THE CARRIZO-WILCOX AQUIFERINTRODUCTION: A WORD ABOUT WATER

Water is the most important resource on the face of our planet. That said, only 2.5% of the water on our blue planet is fresh water. So the fresh water we use every day (and take for granted) is an incredibly precious substance. About 60% of our bodies are composed of water and water is involved in every process in our bodies and in every cell. The source of this precious substance is different depending on where you live. Some places pump water from lakes and rivers and treat it to use as drinking water. In Texas, much of the water comes from the ground in the form of groundwater from aquifers. This report will concentrate on one of the most prolific aquifers in Texas, the Carrizo-Wilcox aquifer. Just as geology is the study of the earth, hydrogeology is the science of groundwater from aquifers beneath the earth’s surface. This report will use the science of hydrogeology to examine the location, origins, age, and uses of the Carrizo-Wilcox aquifer, which will be called the C-W aquifer for convenience. References used in this report are listed at the end in alphabetical order by the author’s last name.WHAT IS IT?

An aquifer is a rock or sediment beneath the ground that is porous (has pores that hold water) and is permeable (has pores that are connected and can transmit water). An aquifer must also have porosity and permeability that can give up that water to pumping for a beneficial use. Examples of an aquifer are sands, sandstone, and limestone.

An aquitard (or aquaclude) is just the opposite of an aquifer. It can, and often does in the case of a clay, contain water, but the pores are not connected so it cannot give up that water for a beneficial use.Examples of an aquitard are clay and shale. The presence of aquitards confines and separates aquifers.

The C-W aquifer is comprised of complex layers of sand, silt, clay, and lignite coal. The Wilcox portion of the aquifer was deposited in a deltaic environment of deposition. This is equivalent to the present-day Mississippi delta.

Complex layers of sand, slit, clay & lignite coal built up over a geologic timescale can be envisioned if we imagine the Mississippi delta, with its fine grained sand channels, wanders back and forth for thousands of years gradually building up layers of sand, silt, clay, and peat. The compression of these sediments by the weight of overlying sediments turns these soft deposits into the sand, slit, clay, and lignite we see today in the Wilcox portion of the C-W aquifer. The Carrizo portion of the C-W aquifer is a little different, because it is composed mostly of medium grained sand. This larger grain size suggests a faster flowing river system which can in turn carry and deposit larger grained sands.WHERE IS IT?

The C-W aquifer extends over all or part of 66 counties stretching from southwest Texas to the Louisiana border. Exhibit 1 shows the location and extent of the C-W aquifer. The solid color on the map shows the recharge zone, where water enters the aquifer, and the subsurface portion is shown with cross- hatching. The recharge and subsurface portion of the aquifer will be defined and discussed later in this report.

Carrizo-WilcoxAquifer

Uvalde

Outcrop

N

1100

Subsurface .. , Miles

WHEN IS IT?The C-W was deposited during the Paleocene Epoch, which ranges in age from 34 to 56

million years ago. This epoch of geologic time occurred just after the Cretaceous era, which was ruled by dinosaurs. By the time the C-W aquifer was deposited, the dinosaurs had already died out and mammals were on the rise. The Paleocene in North America was characterized by a warming trend. The Paleocene Epoch can be placed in the immense reach of geologic time by referencing Exhibit 2. As the illustration shows, the Paleocene Epoch occurred relatively recently in the context of geologic time.

EXHIBIT

2

HOW WAS IT FORMED?The C-W was deposited in two different environments; the Wilcox portion of the aquifer was

deposited in a deltaic environment (similar to the present-day Mississippi) and the Carrizo portion was deposited by fast flowing rivers carrying larger grained sand. The individual sand, silt, and clay layers were laid down within the streambed (sand and silt) with the overbank deposits being clay and lignite (equivalent to the clay and peat bogs on either side of the channel). As geologic time ticks on, the sediments below gradually compact and the position of the channel changes in response to gravity because the water would rather flow towards the lowest elevation. This moves the entire delta sideways and this repeated process builds up the sand, silt and clay layers we see today in the subsurface.

One concept to remember is the principle of superposition; that is, the rock or sediment on the bottom is the oldest with progressively younger rock or sediments deposited above it. That is the foundational principle behind the science of stratigraphy, which studies layers of rock or sediment and assigns them names such as groups, formations, and members (going from largest to smallest or thickest to thinnest). This study of the sediments and rocks in many different places builds up a picture called a stratigraphic column. The stratigraphic column for the C-W is shown on Exhibit 3.

Next we must consider the structure or differences in elevation across a region where sediments are being deposited as well as structural features as a result of deposition such as faulting. This results in the science of structural geology. Some sense of the structures resulting from deposition can be seen by studying Exhibit 4. This page shows the different structural features present during the deposition of the sediments that make up the C-W. The San Marcos Arch is a high spot in the subsurface which separates the environment into two basins; one to the northeast (the Houston Embayment) and one to the southwest (the Rio Grande Embayment). The rivers depositing deltas flowed into these sloped basins, and as we discussed earlier, wandered back and forth depositing sediment. This overall structure results in the dipping of the individual beds to the southeast and southwest as they are laid down in the basins.

A convenient way to visualize this is to draw a line oriented northwest-southeast on a present-day map and fill in the layers or formations present below that line much as you would record the various layers in vertical drill holes along that line. This results in what we call a cross-section. Exhibit 5 shows a northwest- southeast oriented cross-section, (B-B’) which is what the ground would look like if you could cut away the ground and look at it from the side, much as a cut layer cake shows the layers. Cross-section B-B’ shows the characteristic dip or slant of the C-W aquifer as you go from northwest to southeast.WHERE DOES THE WATER COME FROM?

The water in the C-W comes from two basic sources; rainfall and rivers/streams. Looking at the illustration below, where the sand layers come to the surface is called the outcrop. This is also called the recharge zone because this is the location on the surface where the water recharges the aquifer. When a stream or river flows across the recharge zone, some of its water sinks down into the sand and flows down into the aquifer due to gravity. In a similar way, precipitation hits the sandy soils of the recharge zone and sink into the aquifer due to gravity.HOW DOES THE WATER MOVE?

The rainfall and river/stream baseflow sinks into the recharge zone of the aquifer at the surface of the ground and flows downward due to gravity. Wells and springflow pump water from the aquifer and so the water flows downward from the recharge zone to the subsurface in a continuous cycle. Aquitards (non-aquifer rocks or sediments) confine the groundwater to a certain horizon or layer. Drilling into an aquifer layer is how we get the groundwater out of the aquifer to use.

Series South Texas Central Texas Sabine Uplift

Eocene

Claiborne Group

Wilcox Group

Weches Fm. Weches Fm. Weches Fm.

Queen City Sand Queen City Sand Queen City Sand

Reklaw Fm. Reklaw Fm. Reklaw Fm.

Carrizo iUpperSand Wilcox Carrizo Sand Carrizo Sand

Middle Wilcox Calvert Bluff Fm. Upper Wilcox

Paleocene

Simsboi:o Fm. Middle WilcoxLower Wilcox Hooper Fm. Lower Wilcox

Midway Group Midway Group Midway Group

Carrizo-Wilcox Aquifer

Figure 3-1. Paleocene to lower Eocene stratigraphic relationships (modified from Mace and others, 2000) .

Fm=formation

Figure 3-2. Structural elements of the Texas Coastal Plain (modified from Ayers and others, 1985).

Carrizo-Wilcox AquiferOutcrop

!"': :: Downdip

C

[jj

>

-....

--$ -1,0000 -2,000ca>Q)

-3,000

-4,000

B B'0

-2,000

A'Q)

-4,0000caQ)

w-6,000

-8,000

D Younger sedimentsD Sparta Sand•Weches FormationD Queen City Sand•Reklaw Formation

Carrizo SandD Wilcox Group• Carrizo-Wilcox Aquifer

Structural cross section of the Carrizo-Wilcox Aquifer and overlying strata {modified from Kelly and others, 2004).

Miles20

A'A0

HOW MUCH IS THERE?There is a lot of water in the C-W aquifer, but no one has estimated how much is in the

entire aquifer. Consider that the area of the aquifer in the subsurface (where most of the groundwater wells are) is 25,409 square miles (according to George, Mace, & Petrossian (2011)), which is a vast swath of territory. Using this surface area with a conservative aquifer thickness of 750 feet and a porosity figure of 20% gives us approximately 722 cubic miles of water! Dutton (1999) estimated that one sub-aquifer in the C-W, called the Simsboro Formation, in only a four county area contains approximately 170 million acre-feet or a staggering5.5 trillion gallons of water.

WHO USES IT?To understand who uses the aquifer it useful to study the chart below, which represents

what the TWDB estimated groundwater pumpage in 2011 and Exhibit 6, which represents the estimated groundwater pumpage in 1992. The chart below is in acre-feet. An acre-foot is the amount of water that will cover an area 1 acre in size to a depth of one foot; one acre-foot equals 325,851 gallons of water.

It is obvious that the vast majority of the estimated groundwater pumpage is used for the irrigation of plant crops. It is also obvious that the estimated irrigation groundwater pumpage has increased significantly while the amount of municipal pumping for drinking water supplies has actually shrunk. This increase in irrigation pumpage probably reflects the trend towards drier weather we have seen over the last decade and the significant drought we are currently in. This has resulted in a greater amount of water being pumped to make up for the lack of precipitation.

Who uses the aquifer also varies depending on where you are on the aquifer. In a computer simulation, George, Mace, & Petrossian (2011) shows groundwater declines in feet in Exhibit 7. This represents the decline in feet due to groundwater pumpage in the C-W. In the southwest portion of the C-W aquifer is the Winter Garden area of Texas that has intensive raising of crop plants and the groundwater

Municipal Manufacturing Mining Electric Irrigation Livestock

517604

1523812810 1975317242 1832

ESTIMATED GROUNDWATER PUMPAGE 2011

Manufacturing( 3 .5%)

Livestock( 1 .9%) Mining

( 1.8%)

Municipal( 1 6.8%)

Power( .6%)

Irrigation(75 .4%)

Northern drawdown (feet)50

480

Southern drawdown (feet)50

declines in that area are due to groundwater pumpage for irrigation. The low areas in the northeast portion of the C-W aquifer are due to municipal pumping for drinking water usage.HOW PURE IS IT?

To discuss water quality, we must first understand some simple chemistry. The chief measure of groundwater quality is total dissolved solids or TDS. This represents the amount or concentration of ions dissolved in the water. Common ions include chloride, magnesium, carbonate, and others. Generally speaking, the higher the concentration of calcium and carbonate ions in the water, the harder the water is. This is generally not true of C-W water which tends to be low in calcium and carbonate ions or is a softer water. The amount of ions dissolved in water as measured by TDS is in parts per million. One way to envision this is to fill the Alamodome in San Antonio, Texas with basketballs, which would be about 1 million basketballs. If the TDS level is 200 parts per million, then imagine 200 red basketballs mixed among the million regular basketballs in this large arena. Groundwater in Texas is considered fresh if it is less than 1000 parts per million TDS, slightly saline at 1,000 to 3,000 parts per million, moderately saline at 3,000 to 10,000 parts per million and very saline at 10,000 to 35,000 parts per million. Sea water, by comparison is around 30,000 to 40,000 parts per million TDS. The water quality requirements in the Texas Commission on Environmental Quality (TCEQ) for TDS is that it be less than 1000 parts per million TDS.

Although groundwater quality varies throughout the C-W aquifer, it is generally considered fresh, drinkable water. Private water wells tap into the sand layers and pump out fresh, drinkable water that doesn’t need to be treated. However, as you go deeper and deeper into the aquifer, the TDS values tend to go up.This is because there is more circulation in the shallower portions of the aquifer, more fresh water from the recharge zone, and less time for the water to dissolve ions from the sand, silt, and clays. For example, the larger grain size in the Carrizo aquifer produces larger pores (porosity) and makes the sand more permeable (permeability). This greater permeability in the Carrizo portion of the aquifer allows more circulation to bring more fresh water deeper into the aquifer than in the Wilcox portion of the aquifer. It is not unusual for water wells in the Carrizo to produce fresh water at depths of 1000 feet or more.HOW OLD IS THE WATER?

In his study of the age of the groundwater in the C-W aquifer, Boghici (2009) used radiocarbon and tritium dating techniques to determine the ages of the groundwater in wells. He found that the groundwater ranges in age from zero at the outcrop (due to recharge by fresh water) to as much as 27,000 years old in the deeper, downdip portions of the aquifer where circulation is slow.

REFERENCES

Ashworth, John B. and Hopkins, Janie, 1995, Aquifers of Texas: Texas Water Development Board Report 345, Austin, Texas, November, 1995. www.twdb.state.tx.us .

Boghici, Radu, 2009, Water Quality in the Carrizo-Wilcox Aquifer, 1990-2006 : Texas Water Development Board Report 372, Austin, Texas, January, 2009. www.twdb.state.tx.us

Dutton, Alan R., 1999, Groundwater Availability in the Carrizo-Wilcox Aquifer in Central Texas – Numerical Simulations of 2000 through 2050 Withdrawl Projections: Bureau of Economic Geology, Report of Investigations No. 256, The University of Texas at Austin, Austin Texas. www.beg.utexas.edu

George, Peter G., Mace, Robert E., and Petrossian, Rima, 2011, Aquifers of Texas: Texas Water Development Board Report 380, Austin, Texas, July, 2011. www.twdb.state.tx.us

Graham, Joseph, Newman, William, and Stacy, John, 2008, The Geologic Time Spiral – A Path to the Past , United States Geological Survey General Information Product 58.

Klemt, William B., Duffin, Gail L., and Elder, Glenward R., 1976, Ground-water Resources of the Carrizo Aquifer in the Winter Garden Area of Texas, Volume 1 : Texas Water Development Board, Austin, Texas, September, 1976, Second Printing November, 1981. www.twdb.state.tx.us

Klemt, William B., Duffin, Gail L., and Elder, Glenward R., 1977, Ground-water Resources of the Carrizo Aquifer in the Winter Garden Area of Texas, Volume 2 : Texas Water Development Board, Austin, Texas, April, 1977. www.twdb.state.tx.us

Pearson, F.J., Jr., and White, D.E.., 1967, Carbon-14 Ages and Flow Rates of Water in Carrizo Sand, Atascosa County, Texas : Water Resources Research, V. 3, No. 1, p. 251-261.

Thorkildsen, David and Price, Robert D., 1991, Groundwater Resources of the Carrizo-Wilcox Aquifer in the Central Texas Region : Texas Water Development Board Report 332, Austin, Texas, September, 1991. www.twdb.state.tx.us

Thorkildsen, David , 2014, personal communication, August 22, 2014.

TWDB, 2011 http://www.twdb.texas.gov/waterplanning/waterusesurvey/historical-pumpage.asp