Geochemistry of Construction Dewatering Discharge

Impacted by Freshly Poured Concrete

NAHYAN MUHAMMAD RANA

3A Earth and Environmental Sciences – Geosciences Specialization

University of Waterloo

UNIVERSITY OF WATERLOO

Faculty of Science

ENVIRONMENTAL FATE OF CONSTRUCTION DEWATERING DISCHARGE

IMPACTED BY CONCRETE

MMM Group Limited

Kitchener, Ontario

Nahyan Muhammad Rana

3A Environmental Science (Geosciences Specialization)

ID 20486918

August 28, 2015

130 Lincoln Road Waterloo, Ontario N2J 4N3 June 10, 2015 Mr. William Taylor, Department Chair, Department of Earth and Environmental Sciences, University of Waterloo N2J 3G1 Dear Mr. Taylor: This report, entitled “Environmental Fate of Construction Dewatering Discharge Impacted by Concrete” was prepared as my 3A work report for MMM Group Limited. This is my second work term report. The purpose of this report is to introduce and analyse the environmental problem and fate of construction dewatering discharge water that has been in contact with recently poured concrete. MMM Group Limited is a multidisciplinary consulting firm that provides a wide range of civil engineering and environmental engineering services which include, but are not limited to, hydrogeology, transport management, GIS services, environmental planning, water resource management, community design and infrastructure development. The Environmental Management Division, in which I was employed as a Hydrogeological Assistant, is managed by Peter Hayes. Our work consisted of working with clients to perform groundwater studies (surface water and groundwater monitoring), private water well surveys (water quality assessments), erosion and sediment control inspections, and apply for regulatory permits, including Ontario Ministry of the Environment and Climate Change Permit-To-Take-Water (MOECC PTTW) applications, amongst other activities. This report was written entirely by me and has not received any previous academic credit at this or any other institution. I would like to thank Mr. Peter van Driel (supervisor) for his support and guidance and for proof-reading my report. I received no other assistance. Sincerely,

Nahyan Rana 20486918

TABLE OF CONTENTS

1.0 Introduction .......................................................................................................................................... 1

2.0 Construction Dewatering .................................................................................................................... 2

3.0 Regulations ............................................................................................................................................ 4

4.0 Production of Cement and Concrete ................................................................................................ 5

5.0 Cement and carbonate Geochemistry ............................................................................................... 7

5.1 Geochemical Properties of Cement ..................................................................................... 7

5.2 carbonate-carbon dioxide-H2O System .............................................................................. 9

6.0 Natural Methods of Attenuation of High-pH Impacted Water .................................................. 11

6.1 Release to Surface Waters .................................................................................................... 11

6.2 Access to Free Atmospheric carbon dioxide .................................................................... 12

6.2 Re-infiltration to Ground/Groundwater .......................................................................... 14

7.0 Management Strategies/Technologies ............................................................................................ 17

8.0 Case Study: High-pH Discharge out of Sanitary Sewer Manholes in Road and Infrastructure

Construction Project ....................................................................................................................................... 19

9.0 Conclusion ........................................................................................................................................... 20

10.0 Recommendations .............................................................................................................................. 21

11.0 References ........................................................................................................................................... 22

12.0 Appendices .......................................................................................................................................... 23

LIST OF TABLES AND FIGURES

Table 1: Major compounds in Portland cement ............................................................................................ 8

Figure 1: Illustration of Construction Dewatering ........................................................................................ 3

Figure 2: Rotary Kiln ......................................................................................................................................... 6

Figure 3: Freshly Poured Concrete .................................................................................................................. 6

Figure 4: Summary of Concrete Production Process ................................................................................... 7

Figure 5: Stages of Hydration in Curing Process of Concrete .................................................................... 9

Figure 6: Carbonate Ionic Species vs. pH .................................................................................................... 11

Figure 7: Bicarbonate/Carbonate vs. pH ..................................................................................................... 12

Figure 8: Alkaline water treatment with CO2 .............................................................................................. 14

Figure 9: CO2 Concentrations in Soil Air vs. Atmospheric Air ............................................................... 15

Figure 10: CO2 Concentrations in Soil Gases with Respect to Seasonal Variations ............................. 16

Figure 11: Comparison of CO2 vs. H2SO4 in Neutralization of Alkaline Water .................................. 17

APPENDICES

Appendix A: Case Study: Initial + Final Conditions

Appendix B: Site Photographs (June 1 – June 14, 2015)

Appendix C: Table 2: Water Quality Results from Manhole 1

SUMMARY

The purpose of this report is to introduce the environmental problem of dewatering discharge water

coming into contact with high-pH concrete dust at construction sites, before releasing the alkaline

mixture into storm sewer inlets or surface water bodies without proper management or treatment.

First, the concept of dewatering discharge is introduced, followed by a summary of regulation by-

laws in Southern Ontario that serve to ensure a reasonable and adequate standard of water quality

and quantity at construction sites. Next, the chemistry of cement and concrete is analyzed in order

to determine and analyze the compounds responsible for the naturally-high pH of concrete dust.

Furthermore, the fundamentals of carbonate geochemistry will be used to prove that carbon dioxide

can be used to remediate the pH of alkaline waters, a concept which can be (and has been) applied

in the real world as well. Lowering the pH of alkaline waters can be done through either allowing

exposure of the water to free carbon dioxide from the atmosphere or allowing the water to re-

infiltrate into the ground, where higher carbon dioxide concentrations are present (compared to

atmospheric carbon dioxide concentrations). Releasing alkaline waters to surface waters is also an

option, but should be considered as a ‘last resort’ option, as visible calcium carbonate precipitate

plumes can provide an unaesthetic appearance (whitish plumes) to the receiving body of water. This

will be further discussed in the Case Study, which involves a similar environmental problem that was

encountered at a construction site during the spring of 2015. Management and treatment strategies

will also be discussed and recommended, such as using properly managed dewatering discharge

containers and applying concrete dust further away from water bodies.

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1.0 INTRODUCTION

Construction dewatering is a technique widely utilized to remove groundwater or surface water from

trenches and/or excavations in order to provide safer (and drier) working environments for the on-

site workers. Under most circumstances, using a pump or a gravity flume pipe, the water is

transferred from the working area to a dewatering discharge water containment tank, before

discharging the water into a storm sewer inlet or a surface water body, assuming that the water is

free of sediment (low in turbidity and total suspended solids) and/or contamination. Otherwise, the

water would need to be treated accordingly.

On-site cement and concrete works, including pouring concrete dust in preparation for building

infrastructure (i.e. roads, bridge abutments, etc.), are also extensive in almost every infrastructure

construction project. The chemistry of cement is predominantly calcium silicate compounds. During

the process of curing cement into concrete, the cement is hardened by adding water, which

promotes the production of lime [Ca(OH)2], an extremely alkaline (high pH) compound, from the

hydration of the calcium silicate compounds. The result of this process is the production of concrete

dust, which remains alkaline with a pH of above 12.

Therefore, it must be ensured that the poured concrete does not come into contact with surface

water and/or groundwater originating from the Site, including dewatering discharge water, as per

regulation by-laws in Southern Ontario. However, environmental problems have been caused at

several sites due to dewatering discharge water coming into contact with recently poured concrete

dust during surface runoff following heavy rain events, thus elevating its pH to over 12. This report

presents detailed geochemical analysis of concrete and alkaline water, especially limewater (discharge

water mixed with concrete dust), with the support of a project case study (with a similar

environmental problem) based on a construction site in Southern Ontario.

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2.0 CONSTRUCTION DEWATERING

Our understanding of environmental impacts of construction projects, including construction

dewatering, has significantly improved over the past few decades. This is primarily due to the swift

rise in urbanization in North America, as urban centers have increased in size and related

infrastructure (roads, railroads, pipelines, utilities) needed to support the urban centers which have

been built. The natural landscape of Northern and Central portions of North America has been

geologically shaped by prolonged periods of glaciation (glacial cycles). The last glacial period ended

about 15,000 years ago, during the Pleistocene Epoch. These ‘ice ages’ produced mass sheets of ice

that covered the better part of the entire continent, before subsequent warmer periods

(interglaciation periods) caused large-scale melting. Some of the resulting meltwater formed many of

the natural watercourses (lakes, rivers, etc.) that we see in North America today, including the Great

Lakes. Most of this resulting meltwater, however, infiltrated into the porous soils to settle

underground as an aquifer, a groundwater system (Severinghaus, 1999).

To support the rapid rise in urbanization in North America, including Southern Ontario, countless

civil construction projects have been undertaken, involving working below the groundwater level.

Engineers, builders and scientists have encounter groundwater management challenges in order to

complete construction projects. The presence of groundwater at a construction site can affect the

design of the infrastructure, the construction procedures and timelines, and the resulting overall

project costs (Powers, 1992). In order to counter these obstacles, there is a need to gather further

knowledge regarding geology and groundwater conditions (i.e. hydrogeology) and the process of

pumping out groundwater and/or surface water out of excavations (dewatering), either through

pumping or via passive drainage methods (i.e. gravity flume pipe)’ (Powers, 1992). In construction

projects, dewatering has proved to be a useful tool especially in extracting and transferring water

from trenches or excavations (below-ground working areas), which are deeper than the groundwater

Page | 3

table, or become flooded by surface water drainage. In this case, the purpose of dewatering is to

pump the water out at a sufficient rate that keeps excavations and below-ground working areas dry

(Figure 1), thus providing a safer workplace for the workers, and enabling the construction work to

proceed. In some cases, dewatering is permanently required to maintain dry conditions in a low area,

such as a basement, subway, underpass, tunnel, or underground parking garage.

Figure 1: Simple illustration of construction dewatering activities requiring a local ‘drawdown’ of the water table.

Source: http://middourconsulting.com/

Surface water taking (or dewatering) includes pumping out rainwater or floodwater from excavations

following heavy rain events, as well as diverting the flow of water in rivers, creeks, streams or lakes,

around work areas or out of cofferdams, in order to keep the working area sufficiently dry and safe.

An example of when surface water dewatering would be necessary is a bridge construction project

spanning a river, with in-water piers or abutments.

The accumulated water in excavations or cofferdams (in-water work areas) is generally removed

through the use of water pumps, and the pumped water is then released (discharged) to an

appropriate location, which is normally a nearby lake, river, wetland, a vegetated field (where re-

infiltration occurs) or directly into storm sewer inlets (Powers, 1992).

Open Excavation

Dewatering Wells Dewatering water

discharged to a

nearby watercourse

or storm sewer, or

to a vegetated field

where it will re-

infiltrate back down

to the water table.

Page | 4

3.0 REGULATIONS

Groundwater or surface water dewatering is generally required in the construction industry when

excavations are required below the water table, and/or to remove accumulated surface water. For

projects in Ontario, under the Ontario Water Resources Act (OWRA) (Section 34) and the Water

Taking Regulation (Ontario Regulations 387/04), approval from the Ministry of the Environment

and Climate Change (MOECC) is required when the groundwater or surface water dewatering rates

exceed 50,000 L/day (MOECC, 2005), in which case, the applicant must apply for a Permit-to-Take-

Water (PTTW).

According to the MOECC, groundwater and surface water quality must also be monitored and

managed at construction sites involving dewatering, especially those when a PTTW has been

obtained. With respect to surface water quality, the objective is to ensure that the water quality is

satisfactory for aquatic life and recreational purposes, and not compromised by construction

activities. The Provincial Water Quality Objectives are numerical and narrative criteria which serve

as chemical and physical indicators defining acceptable water quality conditions for surface waters of

the Province (MOE PWQO Guidelines and Policies, 1994). In the same way, Ontario Regulations

(O. Reg.) 153/04, Record of Site Condition (amended in April 2011), provides guidelines for

groundwater quality of the Province of Ontario.

When construction dewatering discharge water is being released into storm sewers or sanitary

sewers, the local Municipal or Regional Storm Sewer Use By-Laws must be consulted, and it must be

ensured that the water quality and quantity entering the storm or sanitary sewer does not exceed the

levels indicated in the storm or sanitary sewer use regulations. For example, if dewatering discharge

water were to be released into a storm sewer in the Regional Municipality of York (York Region),

the York Region Sewer Use By-Law would be in effect, whereby the discharge entering the storm

sewer must not, in minimum, have a pH less than 6 or greater than 10.5, a temperature greater than

Page | 5

60 degrees Celsius, and a Total Suspended Solids (TSS) level of 15 mg/L (the water must not appear

to be cloudy or murky).

As shown in Figure 1 earlier, when dewatering operations are taking place, the groundwater table is

locally lowered and, on a wider scale, this forms a ‘drawdown cone’ (a zone of influence due to

dewatering). During construction dewatering works, if a residential property that uses well water

(groundwater) as their primary water supply is estimated to fall within the zone of influence, then

their water quality and quantity is monitored on a regular basis, to ensure that the dewatering works

are not negatively impacting their water supply or quality. In such cases, when groundwater is being

utilized for consumptive (drinking) purposes, the Ontario Drinking Water Quality Standard (O. Reg.

169/03 – ODWQS) is used to indicate acceptable vs. unacceptable water quality for potable use.

The ODWQS ensures that existing water supplies (i.e. private water wells) are not compromised in

water quantity or quality by construction projects occurring at a later time.

4.0 PRODUCTION OF CEMENT AND CONCRETE

Civil construction projects often involve use of large quantities of Cement/Concrete. Cement is the

main ingredient of concrete. Although there are other kinds of cement for various purposes, this

section will solely focus on Portland cement (will be simply referred to as cement from here on), the

most common type of cement. Four essential elements are required to make cement: calcium,

Silicon, aluminum and Iron. The dominant component, calcium, is usually extracted from crushed

limestone, which itself is quarried and blasted for this purpose, while Silicon is obtained from sand

and/or clay minerals, and sometimes Shale. Aluminum and iron are extracted from bauxite and iron

ores, respectively, and are also found in clay minerals. This group is then mixed (ground) together to

form a homogeneous powder, before being heated to high temperatures (800 to 1,500 degrees

Celsius) in a rotary kiln, which is a long, inclined, rotating steel cylinder primarily used for ‘cement-

cooking’, as shown in Figure 2 below (University of Illinois).

Page | 6

As this powdered mixture moves down the rotary kiln, and is exposed to higher temperatures, there

are four (4) primary stages of transformation that take place (University of Illinois):

1) Evaporation of free water in the powder;

2) Calcination: decomposition of limestone and clay;

3) Clinkering: initial formation of calcium silicate compounds; and

4) The final product, clinker, is cooled and ground/mixed with minor amounts (around 5%) of

gypsum (CaSO4 2H2O – used to regulate setting) to form the finished cement powder.

Figure 2: A Schematic Diagram of Rotary Kiln Source: http://matse1.matse.illinois.edu/concrete/prin.html

Figure 3: Freshly poured concrete.. Source: https://stevehyde.wordpress.com/2011/07/08/how-to-build-a-road/

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A summary diagram of the entire process is provided as Figure 4 below.

To produce concrete, the cement powder needs to be hardened through curing, a chemical reaction

which results in maximum strength and durability by increasing its structural integrity and reducing

porosity and permeability. Cement hardens (is cured) as a result of hydration. The strength of the

concrete is related to the water-to-cement mass ratio and the curing conditions (i.e. temperature).

The ‘right’ volume of water is critical to the resulting strength of the concrete, as a high water-to-

cement mass ratio yields a low strength concrete with higher porosity. The majority of concrete is

made with a water-to-cement ratio ranging from 0.35 to 0.6 and at temperatures ranging from 10 to

25 degrees Celsius, according to most concrete manufacturing companies (University of Illinois).

5.0 CEMENT AND CARBONATE GEOCHEMISTRY

5.1 Geochemical Properties of Cement

Cement is predominantly composed of calcium silicate compounds (>70% for Portland cement).

The remaining minor compounds are calcium aluminate and calcium aluminoferrite compounds,

Figure 4: Summary of Concrete Production Process Source: http://matse1.matse.illinois.edu/concrete/prin.html

Page | 8

with minor amounts of gypsum. In summary, according to a study by the University of Illinois,

Portland cement consists of the following five (5) major compounds:

Cement Compound Weight

Percentage Chemical Formula

tricalcium silicate 50% Ca3SiO5 or 3CaO.SiO2

dicalcium silicate 25% Ca2SiO4 or 2CaO.SiO2

tricalcium aluminate 10% Ca3Al2O6 or 3CaO .Al2O3

Tetracalcium

aluminoferrite 10% Ca4Al2Fe2O10 or 4CaO.Al2O3.Fe2O3

Gypsum 5% CaSO4.2H2O

During the process of curing cement, upon the addition of water, tricalcium silicate is the first

compound to be hydrated and therefore greatly contributes to the final strength of the concrete. Its

equation is given by the following:

tricalcium silicate + water � calcium silicate hydrate + calcium hydroxide + heat

2 Ca3SiO5 + 7 H2O � 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6 kJ

As shown in Equation 1, the exothermic reaction releases calcium ions and hydroxide ions (OH-),

which contributes to the abrupt rise in pH of water in contact with the cement from near-neutral (7

to 8) to over 12. This reaction continues until the system is saturated in calcium and hydroxide ions,

which is when the calcium hydroxide (Lime) begins to crystallize, along with calcium silicate Hydrate

crystals, which act as ‘seeds’ upon which more calcium silicate Hydrate can form. The crystals grow

thicker at a steady rate, simultaneously, until the water molecules are unable to diffuse through the

thick coating around the calcium silicate Hydrate crystals.

Table 1: The five (5) major compounds in Portland cement

Source: http://matse1.matse.illinois.edu/concrete/prin.html

Equation 1

Page | 9

As illustrated in Figure 4 above, the pore space, which is initially filled with water (blue color), is

decreasing as the calcium silicate Hydrate crystals (yellow) are thickening (pink) with water and

calcium hydroxide. Diagram D shows nearly-hardened cement paste, where the remaining pore

space is primarily calcium hydroxide solution that has still not crystallized. This hydration reaction

continues until all the water has been used up and all compounds have been hydrated. The same

reaction occurs in the case of dicalcium silicate, although at a much slower rate, as dicalcium silicate

is less reactive than tricalcium silicate (University of Illinois).

5.2 carbonate-carbon dioxide-H2O System

As discussed earlier, the release of hydroxide ions during the process of curing cement causes

elevated levels of pH in the cement-water mixture (freshly poured concrete). In construction sites, if

dewatering discharge water were to be in contact with freshly poured concrete (for example, by

surface runoff caused by heavy rain events), its pH would rise as well. This high-pH surface water

would then be discharged (through pipes or as runoff) into a creek or storm sewer without proper

management or attenuation, which is legally unacceptable as per regional by-laws in Ontario.

Therefore, it is essential to understand the geochemistry of carbonate systems, in order to restore

the water quality of high pH water before discharging into creeks or storm sewers.

Figure 5: Schematic illustration of the pores in calcium silicate through the different stages of hydration Source: http://matse1.matse.illinois.edu/concrete/prin.html

Page | 10

Garrels and Christ (1967) discuss carbonate equilibria in detail using several systems including

carbon dioxide, solution and solid phase carbonates (UC Davis).

Case 1: Neutral pH Waters

In open systems, where water with neutral pH (6 - 8) is in contact with free carbon dioxide from the

atmosphere, carbon dioxide dissolves into the water (becomes aqueous) and reacts with water to

form carbonic acid via the following reaction:

CO2 + H2O � H2CO3

The carbonic acid then dissociates into bicarbonate and hydrogen ions via the following reaction,

whereby the release of hydrogen ions acidifies the water (pH goes down):

H2CO3 � HCO3- + H+

In other words, carbon dioxide is effective in reducing pH of water. In fact, this is also why rain

water has a slightly acidic pH (5.6), as it comes into contact with higher concentrations of carbon

dioxide in the atmosphere, thus as the rain water falls down, it progressively becomes more acidic.

Case 2: Alkaline Waters

In the case of alkaline waters (pH > 10) in contact with free carbon dioxide from the atmosphere,

carbon dioxide reacts directly and rapidly with the excess hydroxide ions to produce bicarbonate via

Equation 4 given below. Since the concentration of hydroxide ions is reduced, the pH is also

lowered. CO2 + OH- � HCO3-

Equation 2

Equation 3

Equation 4

Page | 11

As shown in Figure 6 above, carbon dioxide, carbonic acid, bicarbonate, and carbonate are all

forms of the same ion. At acidic pH levels in waters, carbon dioxide is predominant. Most surface

waters, however, have pH levels ranging from 6 to 9, where bicarbonate dominates. If these surface

waters are alkalized (pH > 11), bicarbonate dissociates into carbonate and hydrogen ions (Equation

5), which decreases the pH. Therefore, in the case of alkaline waters, carbonate becomes the

dominant carbonate ionic species (University of California - Davis). This is illustrated in Figure 7 on

the next page.

HCO3- � CO3(2-) + H+

Simultaneously, the carbon dioxide can also react with water to produce carbonic acid, which in turn

will react with the excess hydroxide ions (instead of water, as in Case 1) to produce carbonate and

water, as shown in Equation 5 below.

CO2 + H2O � H2CO3

H2CO3 + 2OH- � 2H2O + CO3(2-)

Figure 6: Dependence of carbonate ion species on pH. Source: http://euanmearns.com/the-carbon-cycle-a-geologists-view/

Equation 6

Equation 5

Page | 12

When pH of the water is too high, the concentration of carbonate is high. If the calcium

concentration is high as well, then it is possible for precipitation of calcium carbonate to occur. In

other words, the solubility and precipitation of calcium carbonate depends on pH, and therefore the

carbonate concentration. Lower pH values lead to higher solubility (and lower precipitation) of

calcium carbonate (and vice versa), since carbon dioxide or bicarbonate is dominant at lower pH

values (Reefkeeping).

This is why driving the pH very high with freshly poured concrete (or limewater) can rapidly

precipitate calcium carbonate, due to the over-saturation of carbonate ions (from the high pH

water), and also likely due to the added calcium concentration (from the concrete – tricalcium

silicate) and alkalinity.

6.0 NATURAL METHODS OF ATTENUATION OF HIGH-PH WATER

In construction sites, there have been numerous cases of dewatering discharge water coming into

contact with freshly poured concrete or concrete dust. This typically occurs if there are no

containment tanks present (to contain the water) and the water is simply allowed to drain/run along

the ground surface. If concrete is being poured near the dewatering discharge location, then it would

be reasonable to assume that a heavy rain event could cause the concrete and discharge water to

Figure 7: Pictorial representation of the relative number of bicarbonate (green) and carbonate (red) ions in solution as a function of pH.

Source: http://reefkeeping.com/issues/2006-06/rhf/#10

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assimilate and mix together to produce high-pH water. Based on observations at numerous sites, the

initial appearance of high-pH water is clear, as the calcium carbonate precipitation has not begun yet.

However, if the alkaline water is allowed to ‘sit’ and settle for a week (like a pond), enough carbon

dioxide will have dissolved into the water and reacted with lime [calcium hydroxide - Ca(OH)2],

which itself is sourced from the concrete (see Equation 1 in page 8), to precipitate calcium carbonate

out of solution.

Ca(OH)2(aq) + CO2(g) � CaCO3 (s) + H2O (l)

The three (3) following sub-sections explore three (3) natural methods that can be used to attenuate

(restore the pH) impacted waters.

6.1 Release to Surface Waters

If high-pH impacted discharge water is directed to a creek or a river by surface runoff, it is assumed

that, due to the increased rate of reaction with dissolved carbon dioxide in the surface water, calcium

carbonate will precipitate out of solution and settle on the bottom of the surface water, like sediment

or silt. This assumption was confirmed when, during this Spring season at a construction site,

impacted discharge water (initially clear) was released into a river and a calcium carbonate precipitate

plume was observed in the river, while the precipitates were settling on the river bed. This will be

further discussed as a case study in Section 8.1.

Equation 7

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6.2 Access to Free Atmospheric carbon dioxide

Figure 8 shows an experiment carried out by chemists from the University of California, where

slightly acidic water (i.e. rainwater) comes into contact with calcium carbonate which immediately

raises the pH by at least 4 units, via the following reaction (University of California – Davis):

CaCO3 � Ca2+ + CO3(2-)

CO3(2-) + H2O � HCO3- + OH-

Calcium carbonate dissociates into calcium ions and carbonic acid, which in turn reacts with water to

produce bicarbonate and hydroxide ions, thus increasing the pH. To return the pH values back to

near-neutral, the solution was bubbled with atmospheric air, which is 0.03% CO2 in concentration.

It took 5 hours to lower the pH to 8.28, after which it appears that the reaction between CO2 and

water and hydroxide ions is occurring at a much slower rate. In other words, CO2 appears to be

Figure 8: Changes in pH of a solution in response to experimental manipulation of the CO2-HCO3-H2O system

Source: http://lawr.ucdavis.edu/classes/ssc102/Section5.pdf

Equation 5

Page | 15

more effective in reducing high pH levels in very alkaline waters (with pH > 9.5), where the

reactions occur at a greater rate, to a slightly alkaline pH (around 8.5 to 9). In order to further

reduce pH, gypsum (CaSO4 2H2O – Hydrated calcium sulphate) was added to increase alkalinity

and hardness by increasing the concentration of calcium ions, which promotes precipitation of

calcium carbonate, while also increasing the sulphate ionic concentration within the solution, causing

an increase in H2SO4 concentration (an acid), thereby lowering the pH to below 8.

The results of this experiment suggest that this method can also be applied in the field. If impacted

dewatering discharge water is found, this water could be transferred to a containment tank where

prolonged exposure (i.e. over a week) to air, which includes carbon dioxide, will lower the pH to

acceptable levels by precipitating calcium carbonate out of solution. The water may also be aerated

to accelerate this process. Gypsum can be used to further lower the pH; otherwise, if concentrations

of all dissolved metals and compounds (such as Volatile Organic Compounds) meet the regulatory

criteria, the water can then discharged into a storm sewer or a creek with no further treatment.

6.3 Re-infiltration to Ground/Groundwater

Carbon dioxide is much more abundant in soils than in the atmosphere. This is because plant roots

respire and produce CO2, along with additional CO2 produced by oxidative decay of organic matter.

As a result, CO2 concentrations in soils are much greater (approx. 10 times) than the atmospheric

concentrations, as shown in Figure 9 below (Ohio State University).

Figure 9: Composition of CO2 concentrations in soil air vs. atmospheric air Source: http://www.oardc.ohio-state.edu/ss540/lectures/SS540Aeration.pdf

Page | 16

Concentrations of CO2 in soil gases are also affected by seasonal variations, as shown in Figure 10

below, where the lack of vegetation in colder regions, such as Northern Canada, produces less CO2

compared to the warm wet regions (i.e. Brazil). Temperate regions, such as Southern Ontario, fall in

between that range. CO2 concentrations also vary with depth, with highest CO2 concentrations

generally found approximately 1 meter below ground surface.

Therefore, it is assumed that if impacted discharge water were to be allowed to re-infiltrate into the

ground, the rate of precipitation of calcium carbonate would be much greater than the rate of

precipitation of calcium carbonate if the impacted discharge water were to be exposed to air, given

that CO2 concentrations in soil gases are, on average, 10 times greater than that in the atmosphere.

Figure 10: CO2 concentrations in soil air, with respect to seasonal variations.

Source: http://www.gly.uga.edu/railsback/Fundamentals/1121WeatheringCO207.pdf

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7.0 MANAGEMENT STRATEGIES/TECHNOLOGIES

The Environmental Protection Agency (EPA) of the United States recommends installing properly

managed concrete washout areas or facilities to safely contain the freshly poured concrete in

prefabricated washout containers that are protected from spills and leaks. This is necessary in

construction sites because, if dewatering activities are occurring nearby concrete washout areas, and

no washout containers are present, then the discharge water will likely come into contact with the

freshly poured concrete as surface runoff following rain events.

To remediate high-pH water, companies recommend use of carbon dioxide or Sulfuric acid

(H2SO4). One company, Advanced Sensor Technologies Inc. (ASTI), suggests using CO2 instead

of H2SO4 for pH control. This is because H2SO4 is deemed to be highly corrosive; therefore it is

potentially dangerous for workers to store and handle. Specialized equipment, pipes and materials

must be present to safely handle the use of H2SO4. Constant maintenance is also required. As a

result, ASTI strongly recommends the use of CO2 as a viable method for pH reduction of alkaline

waters, as it is safer, efficient, easy to use, and environmentally safe (ASTI Sensor).

Figure 11: Graph showing comparison of CO2 vs. Sulfuric acid in neutralization of alkaline water. Source: http://www.astisensor.com/carbon_dioxide_Versus_Mineral_acids.pdf

Comparative Neutralization Curves of High pH Water

Page | 18

ASTI has developed a pH control system, currently in use in the United States. A CO2 diffuser is

installed by means of a pressurized pipe, through which tiny CO2 bubbles are systematically released

into the alkaline water in the containment/storage tank. As discussed earlier, CO2 reacts with H2O

to produce carbonic acid (H2CO3, a weak acid), which dissociates into highly-reactive ions,

hydrogen Ion (H+) and bicarbonate (HCO3-), both of which react with ions responsible for

alkalinity of water, such as Sodium carbonate (Na2CO3), Sodium hydroxide (NaOH, also called

Caustic Soda), and dissolved lime [Ca(OH)2]. A pH probe is also installed downstream of the

injection point, thus measuring pH in the water following the chemical reactions discussed earlier.

As shown in Figure 11 on the previous page, an experiment conducted by ASTI involved 700 mL

of both carbon dioxide and Sulfuric acid being added to water with a pH of 11. Carbon dioxide

reacted with lime to produce precipitated calcium carbonate and the calcium ions reacted with the

sulfate ions to produce Gypsum, which is represented by the steep drop in pH in the graph. This

steep drop and the gradual decline of pH caused by Gypsum and carbon dioxide, respectively, is also

discussed in Figure 8 on page 14.

High-pH impacted water is recommended to be collected and transferred to a containment tank,

where it can be sampled and analyzed for general chemistry, nutrients, Volatile Organic Compounds

(VOC’s), and Petroleum Hydrocarbons (PHC’s), in order to avoid surface runoff of possibly-

contaminated water into surface waters or storm sewers. Following the treatment of pH of the

impacted water, the turbidity and TSS levels must also be monitored to ensure that the precipitate

and sediment settles on the bottom of the container (the water should not appear ‘murky’). If all

criteria meet regulatory standards, then the water can be discharged to a storm sewer or a creek.

Page | 19

8.0 CASE STUDY: HIGH-PH DISCHARGE OUT OF SANITARY SEWER MANHOLES IN ROAD

AND INFRASTRUCTURE CONSTRUCTION PROJECT

During the spring of 2015, MMM Group Limited had been actively involved in weekly erosion and

sediment control inspections and dewatering assessments at various construction sites in Southern

Ontario. One such location has been selected as a project case study in order to introduce and

analyse a real-world environmental problem involving construction dewatering discharge impacted

by recently poured concrete dust, thus creating a high-pH mixture which flowed as surface runoff

into the river to create a visible calcium carbonate precipitate plume. A location schematic diagram

has been created to assist the reader in understanding the Site in more detail, and is provided in

Appendix A. Photographs of this location are provided in Appendix B. The calcium carbonate

precipitate plume was observed in the river on June 1, 2015, after which an investigation was

conducted to determine the source of the calcium carbonate.

Initial conditions (May to June, 2015):

As illustrated in the Location Schematic Diagram, the sanitary sewer wastewater was flowing north

(from Manhole 1 and 2 to Manhole 4), and then west into the Pumping Station area (Manhole 4 to

Manhole 5). South of the railroad tracks, Manhole 2 was situated in a low spot, causing ponded

water to overtop Manhole 2 following heavy rain events. The water in Manhole 1 flows into

Manhole 2 through the sanitary sewer pipe. The accumulated water (mix of groundwater and surface

water) in the general area in or around Manholes 1 and 2 were dewatered (pumped) and discharged

into the woods to the west (Discharge 1), in order to provide safer working areas for the on-site

workers. This dewatering discharge water was allowed to pond and settle in a depression in the

woods, just south of the railroad tracks, in order to let it re-infiltrate into the ground. The pH of this

Page | 20

water was measured to be 12.5 on June 1, 2015, which reduced to 8.8 by June 8, 2015. The electrical

conductivity and turbidity values were also very high (over 4 mS/cm and 1,000 NTU, respectively).

To the north of the railroad tracks, sanitary sewer wastewater was being pumped into Manhole 5,

which was also situated in a low spot. Concrete dust was being poured in the general vicinity, as

shown in the Schematic Location Diagram. Following several heavy rain events during the month of

May to June, concrete dust was washed down (as surface runoff) into Manhole 5, thus mixing into

the existing groundwater and surface water (rain water). This water was being pumped out and into

the Pumping Station ditch (PS ditch), where it was expected to flow through the existing rock check

dams, through the culvert pipe that led to the woods (river bank), where the water was expected to

attenuate by re-infiltrating into the ground/groundwater and seep into the river as subsurface flow.

However, the vegetation in the discharge area was not dense enough to reduce the flow of water

significantly. Therefore, attenuation (re-infiltration) of the water was minimal and the pH remained

high. The water appeared cloudy and calcium carbonate precipitate was observed on the river banks

and on the river, where a whitish plume was observed. The pH of the cloudy water in the woods

was measured to be 11.5 during the first week of June.

The pH of the river upstream of the discharge location was measured to be 8.1, while the pH of the

river downstream was measured to be 8.4, which falls near the upper limit of the PWQO

requirements for surface water quality (i.e. pH of surface waters should fall between 6.5 and 8.5).

Final Conditions (June to July, 2015):

Following the investigation, the high-pH water from Manhole 5 and Pumping Station was no longer

dewatered into the PS ditch. The pumps were now directed towards the woods on the west end of

the Site. This area (Discharge 2) was relatively flat and elevated compared to the rest of the Pumping

Station area, therefore the water was naturally allowed to pond and re-infiltrate into the ground.

Page | 21

Excess water was flowing down a gully and into the adjacent woods to the north, where even

though steeper slopes were present, the thickness and density of the existing vegetation reduced the

flow of water enough to allow it to re-infiltrate into the ground. The precipitate plume was no longer

observed in the river following the implementation of these changes.

The pH of the water in Discharge 2 was measured to be 12.1. The pH of the water in the woods

could not be measured due to dense vegetation. The pH of the existing water in the PS ditch, where

there was no longer any input from the sanitary sewer dewatering, had reduced to 8.3. The check

dams in the ditch were re-enforced and a filter cloth was placed in this general area to filter

sediments out of surface water before entering the ditch and the culvert.

The environmental and working conditions south of the railroad tracks around Manholes 1 and 2

remained similar to the initial conditions. The water in and around Manhole 1 was sampled on four

(4) occasions during this month in order to analyze for general chemistry, total and dissolved metals

and VOC’s. A table of water quality results (Table 2) has been attached to this report as Appendix

C, to analyze the changes in the following chemical parameters during this month:

• pH (Field + Lab)

• Turbidity (Field + Lab)

• Electrical Conductivity (Field + Lab)

• Total Hardness (as CaCO3)

• Total Dissolved Solids

• Total Suspended Solids

• Alkalinity

• Bicarbonate

• Carbonate

• Hydroxide

• Iron

• Aluminum

• Calcium

Page | 22

As per Table 2, the pH of the water in Manhole 1 decreased from 12.51 to 9.1 (12.2 to 7.37 for lab

pH), which is expected as the water was always in contact with atmospheric carbon dioxide (i.e.

open system).

Electrical Conductivity is dependent on the concentration of ions in solution. In other words,

conductivity increases with ionic concentration. Therefore, since prolonged exposure to atmospheric

carbon dioxide had reduced the carbonate and hydroxide ionic concentration, conductivity also

decreased from >4,000 to 519 us/cm (similar to lab results).

Turbidity numerically describes the cloudiness/haziness of a fluid. The cloudiness is usually caused

by floating (unsettled) silt/sediment particles in the fluid (i.e. water). If a cloudy sample of water

were allowed to sit for a week, the turbidity would be expected to decrease as the silt particles would

settle to the bottom. It is suspected that, since the water in Manhole 1 was allowed to sit for weeks,

the calcium carbonate precipitates and silt particles settled down to the bottom of the shaft, thus

decreasing turbidity over time. The spike in turbidity on June 8 could have been caused by initial

formation of precipitate and increased sediment input from surface runoff following heavy rain

events (supported by Total Suspended Solids (TSS) value on the same day).

Hardness and Alkalinity decreased over time due to the increased rate of dissolved carbon dioxide,

which led to a reduction in concentration of carbonate compounds.

Bicarbonate was at its lowest at a pH of over 12 (June 1), as most of the bicarbonate had

dissociated into carbonate and hydrogen ions (as discussed in Section 5). This is confirmed by the

concentration of carbonate on the same day (68,400 ug/L). As pH decreased over time, carbonate

reacted with the excess hydrogen ions to create more bicarbonate. Carbonate (and also calcium)

also decreased due to the calcium carbonate being precipitated out of solution by the introduction of

Page | 23

carbon dioxide. Hydroxide had also significantly decreased due to carbon dioxide reacting with the

hydroxide ions to produce bicarbonate.

Iron and aluminum are both minor components of the chemical makeup of cement/concrete, thus

are included in Table 2. Iron and aluminum appear to be correlated to TSS, as evidenced by the

increase in iron and aluminum (14 June to 24 June) in response to the increase in TSS. Since iron

and aluminum form significant parts of the chemical makeup of sediments (i.e. clay minerals), higher

TSS would lead to higher iron and aluminum, and vice versa. Iron was found to be highly

concentrated when TSS was at its highest (June 8), although it is difficult to ascertain why aluminum

is relatively low on the same day. It is suspected that sediments composed of aluminum may have

been present at lower quantities on that day, as compared to the iron-rich sediments.

Finally, Total Dissolved Solids (TDS) also decreased as the calcium and carbonate precipitated out

of solution, therefore decreasing TDS and increasing TSS. It can be argued that, in this case, calcium

ionic concentrations in the water is related to the TDS values, as decrease in calcium concentrations

led to the decrease in TDS.

Page | 24

9.0 CONCLUSION

The water quality results from Manhole 1 confirm that, if given enough time, high-pH water

impacted by concrete can be treated through natural means by allowing atmospheric carbon dioxide

to dissolve into the solution and decrease the pH by reacting with the hydroxide ions to produce

bicarbonate, which in turn will dissociate into hydrogen ions, thus lowering the pH steadily.

Re-infiltrating alkaline water into the groundwater is also an effective natural treatment strategy, as

the soils will collect contaminants, sediments and ions as the water percolates through the soil,

where carbon dioxide concentrations are found to be, on average, ten (10) times greater than that of

the atmosphere.

Releasing high-pH water to surface water bodies can treat the water through rapid calcium carbonate

precipitation; however, the end product is an unaesthetic (cloudy) appearance of the receiving water

body.

Page | 25

10.0 RECOMMENDATIONS

Dewatering discharge water should be collected and contained in properly sealed containers.

Concrete works, such as pouring high-pH concrete dust, should be carried out at a location where

contact with groundwater, surface water (other than rain water), and dewatering discharge water is

unlikely. Signs should be put in place where concrete works are underway, to ensure better

communication amongst on-site workers.

Treatment strategies should be further investigated in case of similar environmental problems. Cost-

effect studies should be especially considered by those interested.

Finally, dewatering discharge water quality and flow rates should be periodically checked by

hydrogeologists or on-site workers (who have been trained by hydrogeologists to take field

chemistry measurements appropriately), prior to releasing the water into the fields.

Page | 26

11.0 REFERENCES

J. Severinghaus, E. Brook (1999). "Abrupt Climate Change at the End of the Last Glacial Period

Inferred from Trapped Air in Polar Ice". Science 286 (5441): 930–4.

Powers, J. Patrick (1992). Construction dewatering and groundwater control: new methods and

applications. New York City: John Wiley & Sons.

Rodriguez, J. (n.d.). Construction Dewatering - Definition and Description. Retrieved August 10,

2015, from http://construction.about.com/od/Contractors/a/What-Is-Dewatering.htm

(n.d.). Retrieved August 21, 2015, from http://lawr.ucdavis.edu/classes/ssc102/Section5.pdf

(n.d.). Retrieved August 21, 2015, from

http://www.astisensor.com/carbon_dioxide_Versus_Mineral_acids.pdf

A Simplified Guide to the Relationship Between calcium, Alkalinity, Magnesium and pH by Randy

Holmes-Farley - Reefkeeping.com. (n.d.). Retrieved August 27, 2015, from

http://reefkeeping.com/issues/2006-06/rhf/#10

About Middour Consulting. (n.d.). Retrieved August 10, 2015, from

http://middourconsulting.com/

Scientific Principles. (n.d.). Retrieved August 27, 2015, from

http://matse1.matse.illinois.edu/concrete/prin.html

What is cement and how is it made. (n.d.). Retrieved August 13, 2015, from

http://www.buildeazy.com/newplans/eazylist/cement.html

(n.d.). Retrieved August 27, 2015, from http://www.oardc.ohio-

state.edu/ss540/lectures/SS540Aeration.pdf

APPENDIX A

Case Study: Location Schematic Diagram

Existing Road

RIVER

MH1

Pumping

Station

(PS)

MH2

Case StudyLocation Schematic Diagram (Initial Conditions)

MH5

MH3 MH4

CaCO3

precipitate plume

PS Ditch

MH1 + MH2

Discharge

Old

Can

al L

ock

Str

uct

ure

Old

Can

al L

ock

Str

uct

ure

Wet

lan

d

Concrete work

areaculvert

Check

dams

Upstream

Downstream

Existing Road

River

MH1

Pumping

Station

(PS)

MH2

Case StudyLocation Schematic Diagram (Final Conditions)

MH5

MH3 MH4

MH5 + Pump

Station Discharge

PS Ditch

MH1 + MH2

Discharge

Discharge

2

Wet

lan

d

Check dams

Re-infiltrating

into the ground

Concrete

work area

Filter cloth

Sanitary Sewer Manhole

Sanitary Sewer Wastewater Flow Direction

Surface Water/Dewatering Discharge (being

pumped out) Flow Direction

Surface Water Runoff Flow Direction

Surface water flowing into low area

Woods (vegetated area)

Ponded water in woods due to overflow of

dewatering discharge water

Case StudyLegend

Dewatering Discharge Storage Container

Old Canal Lock Structure

APPENDIX B

Case Study: Location Photographs

JUNE 1, 2015

Photograph 1: Cloudy water inside Manhole 1

Photograph 2/3: Cloudy water in PS ditch, with the culvert seen at the far end of the ditch.

Photographs 3-5: Calcium carbonate precipitate plume observed on June 1, 2015 (See Schematic –

Initial Conditions)

JUNE 8, 2015

Photograph 6: Improved conditions at PS ditch, with no further input from the Pumping Station or

Manhole 5.

Photograph 7: Pumping Station (Manhole 5 is to the right, but not seen in the picture)

JUNE 14, 2015

Photograph 8: Manhole 1 and 2 following heavy rain events.

Photograph 9: Discharge 1 area (See Schematic Diagram).

Photographs 10-11: Pumping Station and Discharge 2 area. Water is being pumped from Manhole 5

(left) and Pumping Station (right) into the dewatering discharge storage container, before being

released into the woods.

Manhole 5

Dewatering discharge

being released into the

woods (Discharge 2 area)

Discharge 2

Photographs 12-13: Discharge 2 area, where water is flowing down this gully and into the woods

(bottom picture).

APPENDIX C

Table 2: Manhole 1 Water Quality Results (June 1 to June 24, 2015)

TABLE 1

Water Quality Results from 4 Sampling Events at Manhole 1

Spring 2015 Co-op Work Term Report

1-Jun-15 8-Jun-15 14-Jun-15 24-Jun-15

MH1 MH1 MH1 MH1

Field pH 6.5-8.5 12.51 8.78 9.1 9.1

Field Temperature (Celsius) - 13.1 13.5 16.3 20.5

Field Turbidity NTU 14 1,515 100 121

Field Electrical Conductivity µS/cm >3,999 1,242 1021 519

Electrical Conductivity µS/cm 2 - 4,120 1,630 891 500

Laboratory pH pH Units 6.5-8.5 12.2 8.19 7.67 7.37

Total Hardness (as CaCO3) µg/L 500 - 793,000 421000 102000 55000

Total Dissolved Solids mg/L 20 - 1,420 894 512 290

Alkalinity (as CaCO3) µg/L 5,000 - 612,000 235000 50000 55000

Bicarbonate (as CaCO3) µg/L 5,000 - <5000 235000 50000 55000

Carbonate (as CaCO3) µg/L 5,000 - 68,400 <5000 <5000 <5000

Hydroxide (as CaCO3) µg/L 5,000 - 543,000 <5000 <5000 <5000

Turbidity NTU 0.5 - 38.6 906 158.0 232

Calcium µg/L 100 - 317,000 143000 33400 19300

Aluminum µg/L 4.0 75 <4.0 4.2 64 123

Iron µg/L 10 300 109 2,110 470 750

Total Suspended Solids mg/L 10 - 58 866 91 101

NOTES:

PWQO

O. Reg. 153/04 as amended in 2013.

1100.0

R.D.L.

Water Management Policies, Guidelines, Provincial Water Quality Objectives of the Ontario Ministry of Environmment

and Energy, July 1994, Reprinted 1999.

Ontario Regulation 153/04 as amended by O. Reg. 366/05, O. Reg. 511/09, O. Reg. 179/11 and O. Reg. 333/13:

Records of Site Condition, Part XV.1 of the Act.

Parameter exceeds PWQO criteria

Reported Detection Limit.

Parameter Unit RDL PWQO

Date Sampled

Sample Location