National Water-Quality Assessment (NAWQA) Program
Citation: Welch, A.H., and Stollenwerk, K.G., Nov. 20-23, 2001, In-situ remediation of arsenic in ground water: In Arsenic in the Asia-Pacific Region, Adelaide, Australia, p.67-68.
Remediation of arsenic in ground water has received increased attention in the United States because of the costs that may be required to comply with a lowered drinking water standard, which was 50 ppb from 1942-2000. Removal of arsenic from ground water within an aquifer, or in-situ remediation, can result in significant cost benefits compared with aboveground treatment. Lower costs may be realized because of lower capital and operating costs, a simpler and less-expensive operation, and avoidance of sludge and wastewater disposal (Rott and Friedle 1999).
Design of in-situ remediation requires an understanding of both the geochemistry and hydrology of a ground water system. Based on this understanding, modification of the chemistry of ground water or aquifer materials may be possible in a manner that results in arsenic removal from ground water. Additionally, ground water with high arsenic concentrations flowing into an aquifer with different geochemical characteristics may result in arsenic removal. Accordingly, arsenic removal may be promoted by pumping that results in flow into aquifers with a chemistry that promotes arsenic removal.
In-situ removal has been successful in decreasing arsenic concentrations from ground water containing high concentrations of both arsenic and iron. Pumping water from one well into a second well after adding atmospheric oxygen can result in arsenic removal from the ground water (Rott and Friedle 1999). The recharged well can then be used for water supply. One advantage of this approach is that 10 or more gallons of low-arsenic water can be obtained for each gallon recharged. Additionally, removal efficiency increases with successive cycles of recharge and withdrawal.
HFO (hydrous ferric oxide) appears to be the most important phase responsible for removing the arsenic from the ground water (Appelo and de Vet in preparation). The arsenic removal process associated with iron removal may be described as a series of reactions involving dissolved oxygen, aqueous and exchangeable Fe and other cations, and arsenic. Injection of water containing dissolved oxygen can lead to rapid exchange of Fe2+ for cations in the injected water with subsequent Fe2+ oxidation to form HFO. Upon reversing the flow direction, the injected water has a lower iron concentration. Continued pumping can produce water with a lower iron concentration because Fe2+ is removed by exchange. Arsenic can co-precipitate with the HFO during injection and adsorption onto the HFO during withdrawal.
A second approach that can lead to arsenic removal is lowering of pH where alkaline ground water contains high arsenic concentrations (Welch, Stollenwerk et al. 2000; Welch, Stollenwerk et al. in preparation). In the system studied, ground water had a high pH (about 9.2) and contained > 100 ppb arsenic. The HFO content of the aquifer was increased through the injection of FeCl3 followed by injection of oxic ground water with a lower pH.
Areas of needed research:
Although the effect of some competing anions, such as P, on the efficiency of arsenic removal is well established, the effect of anions such as CO32- is less certain. Additionally, the effect of multiple anions typically found in ground water, such as sulfate, phosphate, and silica under varying pH conditions is not adequately understood.
Altering a hydrologic regime to promote arsenic removal in systems like those found in some parts of the Bengal Delta might be possible. In some parts of the delta, sediments containing iron oxide underlie more reduced sediments containing high arsenic ground water (Foster, Breit et al. 2000). Pumping of the deeper aquifer could bring shallower, high arsenic ground into contact with iron oxide in the deeper aquifer, possibly leading to arsenic removal. This approach may be possible, although considerable research is needed to evaluate the long-term viability of this approach and to identify those parts of the delta where arsenic removal could occur.
Appelo, C.A.J. and W.W.J.M. de Vet (in preparation). Modeling in situ iron removal from groundwater with trace elements such as As. Arsenic in ground water. A.H. Welch and K.G. Stollenwerk eds., Klewer Publishers.
Foster, A.L., G.N. Breit, et al. (2000). In-Situ Identification of Arsenic Species in Soil and Aquifer Sediment from Ramrail, Brahmanbaria, Bangladesh. AGU Fall Annual Meeting.
Rott, U. and M. Friedle (1999). Subterranean removal of arsenic from groundwater. Arsenic exposure and health effects. C.O. Abernathy, W.R. Chappell, and R.L. Calderon eds., Oxford, UK, Elsevier Science Ltd.: 389-396.
Welch, A.H., K.G. Stollenwerk, et al. (2000). Preliminary evaluation of the potential for in-situ arsenic removal from ground water. International Geologic Congress Pre-Congress Workshop, Rio de Janeiro.
Welch, A.H., K.G. Stollenwerk, et al. (in preparation). Potential for In-situ Arsenic Remediation in a fractured, alkaline aquifer. Arsenic in ground water. A.H. Welch and K.G. Stollenwerk eds., Klewer Publishers.