Importance of Disinfection on Arsenic Release from Wells

Abstract: Arsenic levels in well water used for drinking is a major problem for many homeowners in Wisconsin, particularly for those living in the northeast part of the state where groundwater often intersects arsenic laden mineral deposits. Levels can exceed recommended safe (from a health standpoint) levels, often by large amounts. It has been previously reported that oxidation of arsenic containing mineral deposits by air (oxygen) can lead to elevated arsenic levels. Additionally, it is thought that reduction of oxide minerals associated with microbial processes, such as reduction of arsenic-rich iron oxides by iron bacteria within biofilms, can play an important role in the dissolution of arsenic. Anecdotal evidence suggests that well disinfection treatments (e.g., chlorination), often performed to control microbes in wells, may also be playing a role in arsenic release. This could be caused by the oxidizing strength of the disinfectant or other factors. However, the effect on arsenic levels from disinfection practices is poorly understood. Consequently, studies were conducted to gain more information on the effect of different disinfection practices, particularly the effect of different chlorination treatments. These studies included an assessment of (1) a test well subjected to different disinfection treatments studies (sequentially over time), (2) a survey of a large number of wells with arsenic problems, (3) the effect of biofilm (biological encrustation) treatment with an acid surfactant followed by low dose chlorination and (4) the leaching of arsenic from scale or pipe encrustations. Results of these studies showed that disinfection practices generally caused a temporary increase in well water arsenic (probably from the disintegration or dissolution of biofilms/encrustations). However, once flushed (several well volumes pumped out), arsenic levels generally decreased. There was no indication that any of the disinfection practices evaluated caused sustained increases in arsenic in the well water. The well survey showed that certain well drilling techniques, such as wash-rotary drilling and Bradenhead grouting, were associated with lower arsenic levels in wells. Whether the aquifer was confined or unconfined did not seem to affect arsenic levels. Based on the overall result of the study, acid surfactant treatment of wells, followed by low dose chlorination, is the recommended treatment approach in most situations. The acid surfactant treatment is recommended because of its effectiveness in controlling biofilms, which are believed to play an important role in the dissolution of mineral arsenic in many situations. Clearly, the chemical and hydrologic characteristics of a well can be complex and very site specific, but some general approaches to disinfection that minimize arsenic levels in drinking water are emerging.