THE IOWA STATE-WIDE RURAL WELL-WATER SURVEY: WATER-QUALITY DATA: INITIAL ANALYSIS
B.C.Kross, G.R. Hallberg, D.R. Bruner, R.D. Libra, K.D. Rex, L.M.B. Weih. M.E. Vermace, L.F. Burmeister, N.H. Hall, K.L. Cherryholmes, J.K. Johnson, M.I. Selim, B.K. Nations, L.S.Seigley, D.J.Quade, A.G. Dudler, K.D. Sesker, M.A. Culp, C.F. Lynch, H.F.Nicholson, and J.P. Hughes
Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 19 1990, 142 p.
The State-Wide Rural Well-Water Survey (SWRL) was conducted between April 1988 and June 1989 by the Iowa Department of Natural Resources (DNR) and the University of Iowa Center for Health Effects of Environmental Contaminants. SWRL was designed to provide a statistically valid assessment of the proportion of private rural wells and rural Iowa residents affected by various environmental contaminants. The survey was a systematic sample, stratified by rural population density. SWRL demographic data indicate the sample is clearly representative of rural Iowans.
Primary samples were analyzed for total coliform bacteria; nitrate (+nitrite)-N, ammonium-N, and organic-N; major inorganic ions; 27 pesticides, and 5 pesticide metabolites. Existing agency and laboratory USEPA quality assurance, quality control plans were utilized and verified for SWRL. SWRL collected and analyzed 1,048 water samples from 686 sites.
SWRL was conducted during the driest consecutive two-year period on record in Iowa, with precipitation averaging 14 inches below normal. Monitoring studies indicate the drought limited the movement of contaminants to groundwater. Hence, the SWRL results may present a "best-case" water-quality situation because of the temporal coincidence with the drought.
The SWRL data provide a population-based summary of the drinking water used by rural Iowans, and a cross-section of the quality of Iowa groundwater. The variations in water quality exhibited in the SWRL data, both regionally and particularly with depth, show consistent and predictable geochemical patterns, related to contaminant sources, transport, and age effects. Iowa well waters are near neutral and dissolved ions are dominated by calcium, magnesium, bicarbonate, and in some cases, sulfate. Mean concentrations for all ions, except chloride (Cl) and nitrate (nitrate-N) at shallow depths are related to their surficial sources. State-wide, 1.3% of private well waters exceeded the USEPA maximum contaminant level (MCL) for fluoride (F), and 2.5% exceeded the secondary standard of 2 mg/L. Private well water should be analyzed to assess the natural F content before a supplement is prescribed, to avoid problems with dental fluorosis.
About 18% of Iowa's private, rural drinking-water wells contained nitrate-N >10 mg/L, the recommended health advisory level (HAL); 37% of wells have >3 mg/L, typically considered indicative of anthropogenic pollution. Approximately 14% of wells had detections of pesticides: 16 pesticide compounds were detected, including 11 parent compounds and 5 environmental metabolites; 16 pesticides were not detected. Atrazine and its metabolites were found in 8% of wells. Multiple residues were detected in all regions of the state. The mean concentrations were generally <1 ug/L. Lifetime HALs were exceeded in 1.2% of private, rural wells in Iowa.
Approximately 45% of sites were positive for total coliform bacteria. Total coliforms are ubiquitous constituents of soils, surface water, and shallow groundwater and cannot be equated to fecal coliforms. Only 7% of water systems were positive for fecal coliform bacteria. The only sound, general interpretation of a persistent presence of total coliforms is that the water system is allowing interaction with soil, soil-water, shallow groundwater, or possibly surface water. This can indicate that the system is prone to other forms of contamination.
Individually, or in combination, nearly 55% of rural water supplies exhibited total coliform positives, nitrate-N >10 mg/L, and/or pesticide detections. Using fecal coliforms only, this reduces to about 30% of well-water supplies. Based on 1980 Census data, about 130,000 rural Iowa residents consume drinking water from private wells with >10 mg/L nitrate-N; 94,000 use water with one or more pesticides; 5,400 use water with a pesticide concentration above an HAL.
Statistical analyses show significant associations between many water-quality parameters but the associations are not strong predictors based on state-wide data. By far the most significant factor explaining water-quality variations is well depth. An apparent relationship among total coliforms, nitrate-N, and pesticides is primarily a function of their co-occurrence related to well depth. Total coliform bacteria are very poor predictors of these chemical contaminants. If a prediction were based on the presence of total coliform, the probability is better that they would not occur in the water supply.
Consistent relationships among nitrate-N, dissolved oxygen, and ammonium-N with well depth suggest that nitrate reduction and/or denitrifcation occurs with depth in groundwater systems in Iowa. It is not clear from these data if the deeper groundwater system has the capacity to denitrify the nitrate loads currently being delivered to the system, however.
The effects of sinkholes or agricultural drainage wells are not significant in a state-wide context. Sinkholes were identified in the vicinity of only 2.1% of sites and only 0.6% of sites were near agricultural drainage wells (ADW). No sites reporting ADWs had any pesticide detections or nitrate-N >10 mg/L. Non-farm, suburban housing tracts exhibited the most significant association between landuse and water quality; proportionately, these areas show substantially fewer wells with >10 mg/L nitrate-N and total coliform bacteria. Wells <50 feet from septic systems, showed less nitrate and significantly fewer positives for total and fecal coliform bacteria.
Point source problems affect only a small proportion of wells state-wide. Wells located in feedlots showed significantly higher concentrations of nitrate, but not bacteria problems. Such sites comprise only about 3% of wells state-wide, and account for only about 1% of the wells with >10 mg/L, nitrate-N. Sites where herbicides were mixed within 15 feet of the well showed greater pesticide detections, but again the proportion of wells is low, about 3%, state-wide. Wells located within 15 feet of chemical storage and handling areas are uncommon, occurring at <0.6% of rural sites, and none of these wells contained pesticides or nitrate-N >10 mg/L.
About 5.5% of private water wells in Iowa have experienced a spill or back-siphoning accident with pesticides or fertilizers. These sites exhibit a greater proportion of pesticide detections and high nitrate concentrations than average, as expected, but at the majority of sites the pesticides detected were not those involved in the accident. The sites exceeding HALs for pesticides occurred throughout the state. The sites were dominated by shallow wells; one deep well was involved, and this was a point source case which could affect any depth of well. Two of the sites, 25%, are clearly "point source" cases, a spill and back-siphoning accident (alachlor and trifluralin); the majority, 62.5%, are probably nonpoint sources related to pesticide occurrences in shallow groundwater (alachlor and atrazine); 1 case, 12.5%, equivocal (atrazine).
Well depth is a major variable related to well-water quality, affecting the potential for surficial contaminants to enter a well. The degree of contamination is far greater in shallow wells and significant contamination occurs in wells up to approxiamtely 100 feet deep. Wells <100 feet deep comprise 50% of wells state-wide and account for 70% of total coliform positives, 80% of fecal coliform positives, 64% of pesticide detections and total atrazine detections, and 89% of wells with nitrate-N>10 mg/L. In NE Iowa the depth of contamination is greater because of the deeper groundwater circulation. The greatest proportions of contaminated wells occur in the SC, SW, and NW regions, paralleling regional dependance on shallow wells. In these regions nearly 75% of wells are <100 feet deep (dominantly seepage wells) because alternative water sources are limited.
Certain factors of well construction or placement may afford easy entry of shallow, contaminated groundwater. But these factors are not causes of contamination; if the contaminants were not in the environment they would not get into the soil water and groundwater, or the well. Remediations of well construction or replacing current wells with deeper wells would undoubtedly reduce nitrate and pesticide contamination in many locations, but this would not address the cause of the contamination. The sources of contamination must be addressed because these shallow groundwaters will be the recharge for deeper groundwater with time. Sanitary and structural improvements of private water systems are also needed.
Extrapolating from temporal samples provides an upper limit estimate of wells with likely detections, at sometime over the course of a year: 1. wells >10 mg/L nitrate-N approximately 21%; 2. wells with any pesticide detections approximately 30%; 3. wells with atrazine detections approximately 15%. The systematically selected 10% repeat sites provide a consistent representation of the state-wide data, including representative detections of pesticides down to a 1% occurrence interval. These wells can provide a subset for monitoring trends over time.