Groundwater Contamination and Water-Rock ...

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Procedia Earth and Planetary Science 7 (2013) 101 – 104

Water Rock Interaction [WRI 14] Groundwater contamination and water-rock interaction during leakage of industrial waste water into a carbonate aquifer in an arid zone, Israel A. Burga,* I. Gavrielia, a

Geological Survey of Israel, 30 Malkhei Israel St., Jerusalem 95501, Israel.

Abstract

Leakage of contaminated, acidic, and salty waste water from a chemical industrial complex in the Negev desert, Israel, resulted in increased salinity of the water in the regional carbonate aquifer. Changes in concentrations of the major elements and some ionic ratios enabled delineating the mixing process of the waste water in the aquifer. Although the waste water is enriched in trace elements, no noticeable change in their concentrations was detected in the contaminated aquifer. The removal of the trace elements is attributed to adsorption that was enabled by a rise in the pH of the contaminated water as it comes in contact with the carbonate host rock. © Authors. Published by Elsevier B.V. B.V. © 2013 2012The The Authors. Published by Elsevier Selection peer-review underunder responsibility of the Organizing and Scientific Committee of WRI 14of – 2013 Selectionand/or and/or peer-review responsibility of Organizing and Scientific Committee WRI 14 - 2013

Keywords: contamination, salinization, Judea Group, carbonate aquifer, acidity, trace elements.

1. Introduction Contaminated water from a chemical industrial complex in the northern Negev desert, southern Israel (Fig. 1), was allowed to infiltrate into the subsurface for several decades, until the mid 1990s. The infiltrating water differs significantly from the natural groundwater of the regional aquifer in both the salinity and trace elements content. The interaction between the industrial waste water and the host rock in the unsaturated zone as well as between the rock and the mixed water in the aquifer itself resulted in a sharp change in the composition of the groundwater. Here we describe the changes and processes that occurred over the years in the subsurface that are due mainly to water rock interactions and basic chemical processes.

* Corresponding author. Tel.: +972-2-5314294; fax: +972-2-5314332. E-mail address: [email protected].

1878-5220 © 2013 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 – 2013 doi:10.1016/j.proeps.2013.03.156

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A. Burg and I. Gavrieli / Procedia Earth and Planetary Science 7 (2013) 101 – 104

Fig.1. Northern Negev location map. The industrial complex is located south of the city of Arad. Ef’e-13 well and En Bokek spring are located downstream from the complex at a distance of 9 and 15 Km, respectively.

2. Contamination of the Judea Group regional aquifer in the northern Negev desert The Judea Group aquifer in the northern Negev is a 500 m thick sequence of karstic limestones and dolomites interbedded with some marly layers. The salinity of the aquifer, as established in the 1980s, was 500-550 mgCl/L. The natural flow is north-eastwards, towards En Bokek spring which is the only outlet of the aquifer (0.43 106 m3/year on the average). This spring is located in the Dead Sea rift Valley, close to the artificial evaporation ponds of the Dead Sea Works Ltd. (Figure 1). Until the mid 90’s the salinity of the spring was stable at 500-520 mg Cl/L, similar to the salinity of the aquifer upstream. Below the industrial complex the top 350 m are unsaturated. The carbonate sequence dips here northward towards a local synclinal, where a well (Ef’e-13, 460 m deep) was drilled in 1994 to the aquifer. The contaminating water flowed northward in the unsaturated zone along the geological structure until reaching the water table of the regional carbonate aquifer, where mixing occurred. The industrial complex produced waste water at annual volumes and composition which are not fully known. The water was discharged into nearby evaporation ponds, from which it leaked continuously, as well as directly infiltrated into a karstic swallow-hole. Based on some scarce data, Burg and Naor [1] evaluated that approximately 2 106 m3 of water leaked and discharged every year into the subsurface, amounting to a total volume of ~75 106 m3 since the beginning of the industrial activities. The waste water originated from different industrial sources and thus included dissimilar types of contaminants, each source characterized by its own composition (Arad and Halicz [2]; Burg and Naor [1]). Generally, the contaminating waste water was highly saline, particularly enriched with Ca and Cl and often very acidic (pH of 1-2.5). The acidic waters were also characterized by high concentrations of trace elements, including Al, Cd, Zn, Pb, Cu, Cr, As, B, Mo, Ni, Mn, Fe, and PO4. The high salinity and chemical composition of the contaminating water can be appreciated from the composition of the water encountered in a shallow well that was drilled to a small and restricted shallow perched aquifer adjacent to the evaporation ponds (Table 1).


Table 1. Concentrations of major elements measured in a shallow well adjacent to the evaporation pond (measured in 1992-1994).

Element

Concentration (mg/L)

Element

Concentration (mg/L)

Na

2,700 - 3,300

Cl

91,000 - 100,000

Mg

3,000 - 5,400

Br

2,500

Ca

32,000 - 48,000

SO4

1,100 - 1,450

Taking into account the different water sources, Burg and Naor [1] evaluated that the average salinity of the leaking water was at least 40,000 mgCl/L. As compared to the natural uncontaminated water in the Judea Group aquifer, the contaminating water was characterized by very high concentrations of Cl, Na, Mg, Ca, Br, only slightly more concentrated in SO4, and having low Na/Cl (0.15), low Cl/Br (97), very low SO4/Cl (0.02), low Mg/Ca (< 0.2) and very high Ca/HCO3 equivalent ratios. The last ratio is the result of the low pH value of the waste water which decreases the solubility of HCO3. The salinity of the water encounted in the Ef'e-13 well during its pumping test in 1995 was found to be unexpetedly much more saline than the natural background (5,500 vs. 580 mgCl/L, respectively, Figure 2). When the well was resampled four years later the salinity was even higher: 8,500 mgCl/L, with a sharp increase in the other major elements concentration (except for SO4 and HCO3, Figure 2), indicating a contineous salinization and contamination of the groundwater. No data is available from the well after 1999. In agreement with the salinization process in Ef’e-13 well, the salinity of the En Bokek spring also increased drastically, with the chloride concentration increasing by a factor of over the last 18 years, from 500 mg Cl/L in 199 to 3,500 mgCl/L in June 2012. 9,000 E'fe-13, 1995 E'fe-13, 1999

8,000

Judea Group, Natural water

7,000

concentration (mg/L)

6,000

5,000

4,000

3,000

2,000

1,000

0 Ca

Mg

Na

K

Cl

SO4

HCO 3

Br

element

Fig. . Comparison of the major elements concentrations between the natural groundwater in the Judea Group aquifer and the Ef’e-13 contaminated water in 1995 and in 1999.

The salinization of the groundwater at Ef'e-13 can be easily traced to the industrial waste water not only because this is the only source of saline water that exists within the drainage basin, but also through its chemical composition and ionic ratios. This is best illustrated by the rise in the major elements concentrations and change in the ionic ratios that follow the mixing lines between the natural uncontaminated groundwater and the industrial waste water (Fig. 3). Due to the extreme arid climate in

103

104


the drainage basin, there is no dilution of the groundwater by recent recharge and the chemical composition of the groundwater downstream from the contaminated source is determined solely by the mixing between the two sources. ~20% of the groundwater that was pumped in 1999 from Ef’e-13 is attributed to the industrial source. 0.45

0.80

uncontaminated

0.60 0.50 0.40

average composition ot the sewage water (Burg and Naor, 2000)

0.30

uncontaminated

0.40

0.20

SO4/Cl (eqv. ratio)

Na/Cl (eqv. ratio)

0.70

0.35 0.30 0.25 0.20 industrial contaminated water (evaluation, Burg and Naor, 2000)

0.15 0.10

0.10

0.05

0.00 100

1,000

10,000 Cl (mg/l)

100,000

0.00 200

400

600

800

1,000

1,200

1,400

SO4 (mg/l)

Fig. . Ionic ratios in the contaminated water from the Ef’e-13 well. Note that all samples fall on mixing lines between the natural uncontaminated water of the Judea Group aquifer and the average industrial contaminating water.

The acidic nature of the contaminating water is no longer identified in the groundwater. This is due to the contact of the acidic water with the carbonate host rock, which resulted in massive dissolution and increase in the permeability of the rock. However, no noticeable change in the trace elements concentration was detected in the contaminated water in the well (expect for boron) despite their enrichment in the industrial waste water. It is suggested that the rapid rise of the pH of the waste water in the subsurface to values typical of carbonate aquifers (slightly above 7.0) decreased the solubility of the trace elements that were then adsorbed in the host rock. Indeed, Arad and Halitz [2] described similar process in a series of lab experiments during which the trace elements were adsorbed onto iron hydroxides upon contact with the carbonate rock. Thus, by the time the neutralized industrial waste water reach the regional groundwater they contain no elevated levels of trace elements. 3. Summary A prolonged leakage and recharge of contaminated industrial waste water from a chemical industrial complex in the northern Negev desert, Israel, resulted in salinization of the water in the regional carbonate aquifer of the Judea Group. The contamination of the groundwater can be traced to its source through major elements No noticeable change in the trace elements concentration was detected in the contaminated water (except for boron) due to adsorption of these contaminants, most probably onto Feoxy-hydroxides within the aquifer. The trace elements are non-conservative due to water rock interaction in the carbonate aquifer. No dilution of the groundwater by recent recharge occurred, thus the chemical composition of the groundwater downstream from the contaminated source is determined solely by the mixing between two sources: the natural water in the aquifer and the contaminated industrial waste water. Using chemical concentrations as well as relevant ionic ratios enable us to follow the chemical evolution of the groundwater, where salinization and host rock dissolution occurs simultaneously. References [1] Burg A, Naor H. Evidences from Ef’e 13 borehole for pollution of the Judea Group aquifer in the Rotem plain and recommendations for rehabilitation. Tahal, Report 6757-d00.164; 2000, 28p. (in Hebrew). [2] Arad A, Halicz L. Industrial wastes of ‘Israel Chemicals Industries’ in the Rotem Basin. Geol. Surv. Isr., Rep. TR-GSI/16/93; 1992, 12 p. (in Hebrew).