A case study shows regular operation and maintenance can mitigate arsenic issues.
By Ned Marks, PG, and Michael Schnieders, PG, PH-GW
Arsenic is a naturally occurring element found in many rocks and sediments, with natural concentrations of arsenic ranging widely with higher concentrations generally found in igneous and sedimentary rocks associated with iron and manganese.
Arsenopyrite (FeAsS), an iron arsenic sulfide, is the most common mineral source of arsenic according to Los Alamos National Labs. Arsenic is released into the surface water and groundwater supplies through natural processes including erosion, weathering, and dissolution. There are also several anthropogenic sources of arsenic including mining, agriculture, metal manufacturing, and refining.
Historically, arsenic was used as a poison, but the modern concern is its role as a carcinogen. In 2001, the U.S. Environmental Protection Agency lowered the National Primary Drinking Water Regulation (NPDWR) maximum contaminant level for arsenic in public water supplies to 10 micrograms per liter (µg/L) or parts per billion (ppb).
This level also serves as a guideline for acceptable levels in private wells. As with all aspects of the NPDWR, states and tribal entities are responsible for enforcement and can lower the standard.
Some states are faced with elevated levels of naturally occurring arsenic. The U.S. Geological Survey found that arsenic was detected in nearly half of the wells sampled in aquifers used for drinking water supply at a concentration of 1 µg/L or greater.
Bruce Stanton, a professor at the Geisel School of Medicine at Dartmouth College, states the problem may be more pronounced.
“Arsenic can be found in well water at levels above the EPA standard of 10 ppb in as many as one in five private wells in New Hampshire and many other states, including Maine, Michigan, California, New Mexico, Arizona, Colorado, and Nevada,” he says.
The EPA estimated in 2001 the annual cost to reduce arsenic concentrations to below the MCL would range between $0.86 to $32 per household for customers of large public water systems (more than 10,000 people) and $165 to $327 per household for very small systems (25 to 500 people).
Understanding the factors that affect concentrations of arsenic and other contaminants with geologic sources in groundwater can help water suppliers prioritize areas for new groundwater development and reduce treatment costs.
Arsenic is oftentimes a known contaminant in a region, but the rate of occurrence and the concern can vary greatly. Small communities and rural water districts oftentimes live in a mindset that it’s almost “best not to ask” as treating the problem could be cost prohibitive and a new water source may be required.
Over the past decade, consultants with Terrane Resources Co. in Stafford, Kansas, and Water Systems Engineering Inc. in Ottawa, Kansas, have collaborated on well forensic geoscience evaluations—applying investigative processes and deductive reasoning to address well fouling and water quality issues.
One area of concern has been public supply wells with gradual increases in arsenic concentrations which coincided with evidence of fouling and impaction. The work has focused on wells of a common nature within the central states: low carbon steel casing, minimal grout, lack of development, less effective pump configuration, poor maintenance, and operated with a minimum amount of oversight but a lot of wishful thinking. Involvement is primarily driven by a loss of capacity, but aesthetic or actual water quality issues have sometimes initiated the investigation.
Enter Well No. 3
Well No. 3 is a public water supply for a small community. The well serves as part of a four-well system which serves a town. Well No. 3 was originally constructed in 1948 and is shallow in depth with 15-foot of Aramco bronze shutter screen. Despite operating for an extended life, it had not lost much production capability, producing at a rate of 83% of the original at the time of involvement.
Terrane Resources was called in to evaluate Well No. 3 due to a noted rise in arsenic levels. As evident in Figure 1, arsenic levels that had been of little concern for nearly 20 years of required monitoring had begun to climb. In a prudent move, the local city council wanted to evaluate the well’s long-term viability and the likelihood of the city needing to invest in a costly arsenic removal system.
Water samples were collected and submitted to Water Systems Engineering for evaluation. Samples were collected at start-up and after sufficient pumping to clear the well and draw in aquifer water.
The water chemistry for Well No. 3 was somewhat aggressive in nature with negative Langelier Saturation Index (LSI) values recorded. Hardness present was predominantly carbonate based, typical for the region, but due to the negative LSI, carbonate scale was not expected. Iron levels, evaluated in multiple forms, were not excessive although some concentrating was evident. Manganese levels were high, reflecting a known issue in shallow alluvial sediment within the area.
Evaluation of the microbiological community within Well No. 3 was conducted. Testing went beyond the typical total coliform presence/absence to include an evaluation of the total microbial population via heterotrophic plate count (HPC) and adenosine triphosphate (ATP) testing. Additionally, testing for the level of anaerobic growth, presence of sulfate-reducing bacteria, and the presence for iron bacteria were conducted.
The analysis concluded that a larger microbial population was present. The bacterial presence was largely aerobic and contained low to moderate levels of iron- and manganese-oxidizing bacteria. This evaluation process will prove to be insightful as the project progresses.
During the investigative phase, a call was received from the client regarding a “glob of stuff” that had plugged the meter. Samples were retrieved and sent to Water Systems Engineering for evaluation of the material, and specifically, any biological components (Figure 2). Additional samples were extracted from the primary sample and sent to two separate certified labs for total and dissolved arsenic testing.
As you can see from the data in Table 1, the deposit was comprised of a very large, dynamic, and diverse microbial population. Total arsenic levels were at a level considered excessive, far higher than the levels observed in the regular water testing conducted.
In our opinion, the data clearly showed the role that biological deposits play in concentrating arsenic within the lower extension of Well No. 3. These results have helped to direct further sampling and analytical parameters. It is important to understand this is still a small subset of samples and developing hard set rules and trends is not reasonable currently.
Research on arsenic mobilization suggests that the common natural reaction in groundwater is “reductive dissolution,” which means the water in the water/rock relationship would need to be reductive. From the analysis of the ORP levels in Well No. 3, the produced water is not reductive (average 222.8 mV). Furthermore, the ORP readings remained stable with operation. At the other end of this theory is an overly oxidative environment, which again is not apparent in the readings.
Investigative efforts also turned up an older video survey of the well which identified holes within the upper casing. Additionally, the operator provided a large root mass which was removed from the well several years earlier.
Historical research indicated that Well No. 3 was acidized in 2017 to address suspected fouling. The rehabilitation effort utilized a high concentration of an aggressive mineral acid. With a lack of carbonate scale potential in the well, it is suspected that the acid treatment may have contributed to damage of the well casing and some dissolution of silts and clays from the aquifer, plant material, and bacteria-related biofouling.
Whether the heavy acid treatment is the specific cause for the high arsenic is difficult to ascertain. However, we can identify that it was a turning point in the quality of the produced water.
At the point of involvement, we know Well No. 3 was in disarray in several areas of concern: structural damage of the upper casing, breach of the well by vegetative matter (roots), cascading water from a shallow subsurface source with potential direct influence from the surface, high levels of microbial activity, presence of iron and manganese bacteria, and degrading water quality with regards to the arsenic content.
After a review of the current knowns and unknowns, the city council made the decision to move ahead with a cautious and limited treatment work plan, and that with measured success, additional efforts would be employed to maintain Well No. 3. The alternatives were replacement of the well and development of a new groundwater source or employment of an arsenic removal system and shouldering the legacy of handling media and managing discharge.
The first step was removal of the existing pump, column pipe, and shroud. As visible in the first photo above, the column pipe was heavily occluded with biomass, sediment, and what appeared to be larger, vegetative material. Additionally, accumulations of debris were airlifted with an eductor pipe and the broken tail pipe was retrieved with a fishing tool from the bottom of the well.
These first steps confirmed that Well No. 3 was suffering from poor maintenance and a severe lack of TLC. Minimal mechanical cleaning efforts were employed to disrupt and remove fouling present within the well column without jeopardizing the well structure.
Data suggests the removal of the fine-grain sediments from the well also reduces the potential of excessive total arsenic. In attempting to understand these challenges, total versus dissolved arsenic lab results are an informative diagnostic tool.
It was recommended and approved that the well be videoed again to verify the integrity of the now visible lower portion of the screen. Fortunately, the screen and base appeared to be competent.
Prior to installation of the liner, and based on the lab data, the well was treated with a mild organic acid solution with a biodispersant. Gentle brushing was performed as a means of agitation. Treatment chemicals were placed only in the screen interval and the reaction was closely monitored. Brush tooling was restricted to the screen interval. Disruption of the existing casing was kept to a minimum.
Following cleaning, the upper casing section, which exhibited several holes from 22 feet to 26 feet, was lined with Johnson Screens SDR17 Shur-Grip casing. Once installed and grouted in place, the well was disinfected with a pH-adjusted sodium hypochlorite solution of a larger volume to treat the well and adjacent aquifer-interface zone.
The chlorine solution was evacuated, and the well was sampled. Until the sample results were known, the well was pumped approximately one hour per day (to waste) to limit stagnation and the potential for degradation. Idling a well aids in the development of anaerobic conditions and increases in the microbial population which could countermand treatment efforts.
The placement and configuration of the pumping system is an integral part of the well improvement process. If the pumping system is not properly configured to eliminate the anoxic zone, the chance of re-infestation of anaerobic bacteria increases.
Post cleaning and disinfection, the well was regularly tested for total and dissolved arsenic, as well as several microbiological parameters including adenosine triphosphate for total microbial population, presence of iron bacteria, presence of sulfate-reducing bacteria, and the level of anaerobic growth.
A significant decrease in microbial activity occurred following cleaning efforts, with a steady population decline continuing once the well was returned to active use. Similarly, arsenic levels fell from the pre-investigation levels to levels consistently below the NPDWS of 10 ppb.
Multiple Wells, Similar Results
The phenomena observed in Well No. 3 have been repeated in multiple public water supply wells with similar, successful reduction in arsenic.
Research into the conditions and reactions continues. Historically we have seen dirty wells, wells that lack development, or wells that produce fine-grain sediments routinely test positive for total arsenic.
This phenomenon is due to the mineralogy and petrology of the formation materials. The naturally occurring arsenic compounds in the formation are typically not available to be dissolved into the groundwater. But once that piece of sediment is captured in an acidified sample, the arsenic and several other elements can be released and contaminate the sample.
The data is not showing that microbial activity is the sole cause of arsenic mobilization or presence in well systems, but that biofouling and poor maintenance procedures can have an impact on arsenic concentrating downhole.
Sound well design, regular operation, preventative maintenance, and timely response to problems appear to aid in mitigating the arsenic challenge—very similar to many problems occurring downhole.
It’s important to understand that the wealth of the system is dependent on the health of the well. Keeping wells healthy minimizes losses to the system and maximizes cost-effective clean water to the customer.
Ned Marks, PG, is the president of Terrane Resources Co. in Stafford, Kansas. He has worked on many different aspects of groundwater projects including water rights work, work in the mining industry, livestock industry, ag-chemical industry, and public water supplies. His goal is to prevent pollution when possible utilizing source water protection, site appropriate well design and construction, and sound wellfield management. Marks can be reached at email@example.com.
Michael Schnieders, PG, PH-GW, is the president and principal hydrogeologist at Water Systems Engineering Inc. in Ottawa, Kansas. Schnieders was the 2017 NGWA McEllhiney Lecturer in Water Well Technology. He has an extensive background in groundwater geochemistry, geomicrobiology, and water resource investigation and management. He can be reached at firstname.lastname@example.org.