HYDRO-GEOCHEMICAL PROCESSES DRIVING ARSENIC CONCENTRATIONS

IN SPRINGS IN THE THE HIGHLANDS OF COSTA RICA

Glen V. C. Aguilar[1], Daniel Riddle[2], Melissa Zapata[3], Kaya Weatherby[4], Peter S. K.

Knappett[3], Kelly Brumbelow[3], Georgianne Moore[3]

University of Guam[1]; Utah Valley University[2]; Texas A&M University[3]; Boston University[4]

Acknowledgements:Funding for this Research Experiences for Undergraduate program is provided by the National Science Foundation’s Division of

Earth Sciences (EAR-1659848).

References:Knappet, Peter, et. al, 2019. “Processes Driving Rising Arsenic Concentrations in an Intensively Pumped Agricultural Basin in

Central Mexico”.

Kibet, J., et. Al, 2016. “The Geochemical Speciation of Hand-dug Well Water of Kakamega County, Kenya”.

• The objectives of this study are to:

1. assess levels of arsenic (As) and fluoride (F)

in drinking water springs as potential human

health risks in the highlands of Costa Rica;

2. investigate the hydro-geochemical processes

driving observed concentrations across this

region.

We hypothesize that in geothermally active areas,

a gradation of springs exist from purely fresh,

meteoric recharge water with low Total Dissolved

Solids (TDS) to geothermal spring water with high

TDS.

Field Analysis

• Locate springs and map coordinates (Garmin

eTrex 30)

• Test water for temperature, pH, specific

conductance, and ORP (YSI 1030 Pro)

• Test water for alkalinity (HACH Model AL-DT)

• Filter and bottle samples for laboratory analysis

• Samples were then stored at 4°C prior to lab

analysis. • Data supports hypothesis that there is a gradient

between freshwater and geothermal water

• Nitrite and nitrate may not adhere to the gradient due to

them not being part of the biosphere

• We will not yet fully understand hydro-geochemical

processes of the system until water isotopes results are

completed

• Further studies and analysis may explain why certain

parameters decrease and specific conductance and

temperature increase

Figure 1: Sample sites in Costa Rica

METHODOLOGY

INTRODUCTION

%𝐶𝐵𝐸 =Σ 𝑐𝑎𝑡𝑖𝑜𝑛𝑠 −Σ |𝑎𝑛𝑖𝑜𝑛𝑠|

Σ 𝑐𝑎𝑡𝑖𝑜𝑛𝑠+Σ |𝑎𝑛𝑖𝑜𝑛𝑠|∗ 100

RESULTS

Lab Analysis

• Anions and cations were analyzed using ion

chromatography (Dionex 500, Thermo-Fischer

Scientific, Waltham, MA).

• Water isotopes δ18O and δ2H were analyzed on

a Picarro cavity ring down system (Picarro Inc.,

Santa Clara, CA).

• Trace elements were analyzed using Inductively

Coupled Plasma Mass Spectroscopy (ICP-MS)

Figure 4: Piper plot showing major ion proportions in

samples.

Equation 1: Charge Balance Error helps us judge the

validity of water analyses. Ideally CBE is ±5%

• Water chemistry of geothermal springs mostly

consist of sodium chloride

• Treated river water and freshwater springs mostly

consist of magnesium bicarbonate

• Box and whisker plots show there is a gradient in

most of the chemical parameters

• Our bivariate plots show that as specific

conductance increase, many chemical parameters

seem to decrease

DISCUSSION CONCLUSION AND FUTURE RESEARCH

Figure 2: The graphs above show the gradient in field measurements and TDS between the different water types

Figure 3: The graphs above show the relationship between the highest correlating parameters

1

10

100

1000

1 10 100 1000 10000Alk

alin

ity (

mg

/L)

Specific Conductance (𝜇S/cm)

CA

TIO

NS

AN

ION

S

1

10

100

1 10 100 1000 10000

Cal

ciu

m (

mg

/L)

Specific Conductance (µS/cm)

1

10

100

1000

1 10 100 1000 10000

Sod

ium

(m

g/L

)

Specific Conductance (µS/cm)

1

10

100

1 10 100 1000

Cal

ciu

m (

mg

/L)

Alkalinity (mg/L)

0.0001

0.001

0.01

0.1

1

10

1 10 100 1000 10000

Lith

ium

(m

g/L

)

Specific Conductance (µS/cm)

RESULTS

Flu

ori

de

(m

g/L)

00.05

0.10.15

0.20.25

0.30.35

0.40.45

0.5

Ch

lori

de

(mg/

L)0

100

200

300

400

500

600

700

Nit

rite

(m

g/L)

00.20.40.60.8

11.21.41.61.8

2

Sulf

ate

(mg/

L)

020406080

100120140160180200

Bro

mid

e (

mg/

L)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Nit

rate

(m

g/L)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Ph

osp

hat

e (

mg/

L)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Lith

ium

(m

g/L)

0

0.5

1

1.5

2

2.5

Sod

ium

(m

g/L)

0

100

200

300

400

500

600

700

800

Am

mo

niu

m (

mg/

L)

0

2

4

6

8

10

12

14

16

Po

tass

ium

(m

g/L)

0

10

20

30

40

50

60

70

Spec

ific

C

on

du

ctan

ce

(µS/

cm)

0

500

1000

1500

2000

2500

3000

3500

Mag

nes

ium

(m

g/L)

0

10

20

30

40

50

60

70

Cal

ciu

m (

mg/

L)

515

25

35

45

55

6575

85

95

Alk

alin

ity

(mg/

L)

30

50

70

90

110

130

150

170

190

OR

P (

mV

)

100150200250300350400450500550600

pH

0

12

3

4

5

6

7

8

9

Figure 5: Sample sites and field sampling in Costa Rica