THE IOWA STATE-WIDE RURAL WELL-WATER SURVEY: OCTOBER 1990 REPEAT SAMPLING OF THE 10% SUBSET
K.D. Rex, R.D. Libra, G.R. Hallberg, B.C. Kross, R.W. Field, L.A. Etre, L.S. Seigley, M.A. Culp, B.K. Nations, D.J. Quade, J.K. Johnson, H.F. Nicholson, K.L. Cherryholmes and N.H. Hall
Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 25, 1993, 26 p.
The Iowa Department of Natural Resources, in conjunction with the University of Iowa, Center for Health Effects of Environmental Contaminants, conducted the State-Wide Rural Well-Water Survey (SWRL) between April 1988 and June 1989. The SWRL survey systematically selected and sampled 686 sites and provided a statistically valid assessment of the proportion of private rural wells and rural Iowa residents affected by various environmental contaminants. The SWRL design framework also systematically selected a subset of 10% (68) of all sites for a one-time repeat sampling, to assess temporal changes in water quality during the original survey. The 10% repeat sites yielded a very consistent representation of the state-wide data, including proportionately representative detections of pesticides down to about a 1% occurrence interval. These sites provide a representative subset of SWRL for monitoring water quality over time as an indicator of temporal change. The first two samplings of this 10% subset of wells are termed SWRL 10-1 and SWRL 10-2, respectively (abbreviated herein as 10-1 and 10-2). The SWRL 10-1 was part of the full SWRL sampling, and therefore is used as the basis for comparison with subsequent samples.
The SWRL survey was conducted during the driest consecutive two year period in Iowa's recorded history. The objective of this study, the third sampling of the SWRL 10% subset (referred to here as SWRL 10-3 or 10-3), was to resample the subset during more "normal" climatic conditions, and to assess changes in water-quality that may have occurred. The 10-3 sampling was done in October 1990, after weather patterns in Iowa had changed from the drought conditions of 1988-1989, to more normal, to more wetter-than-normal conditions. Long-term monitoring has shown that this mid-fall period is typically "calm," hydrologically, and often represents conditions near the annual average for such parameters as discharge (general water-flux) and nitrate concentrations, though typically fewer pesticide detections occur than in late-spring or summer. For cost and technical reasons there were some differences in the analytes included in SWRL 10-3.
In October, 1990, during SWRL 10-3, about 20% of the sites showed nitrate-N >10 mg/L, almost 50% were positive for total coliform bacteria, 19% positive for fecal coliform bacteria, and 13.5% contained detectable atrazine (the parent active ingredient, no metabolites included). Only 6 other common herbicides were included in the analytes for 10-3; compared to the 17 herbicide active ingredients and 2 metabolites that were included in the full SWRL. None of these other compounds were detected in 10-3, but with the significant increase in atrazine there was not real change in the proportion of wells where any pesticide was detected. The pattern of statistically significant differences in nitrate concentrations, bacteria occurrences, and atrazine detections between wells <100 and >100 feet deep continue to be apparent in the 10-3 sampling.
The only statistically significant changes between the full SWRL (and 10-1) in 1988-1989 and the 10-3 sampling in October 1990, were: 1. the decline in detection of dissolved organic-nitrogen; 2. the increase in fecal coliform positives; and 3. the increase in atrazine detections. Results for nitrate and total coliform bacteria from SWRL 10-3 are statistically similar to prior samples.
Based on the SWRL 10-3 sampling, a relatively small subset sample of comparative well and kitchen tap samples (60 wells and 31 kitchen taps), no problems with lead in Iowa's rural drinking water-supplies were discerned. Lead concentrations in rural well-and tap-water are generally below 1 µg/L. No samples analyzed contained lead at a concentration greater than 15 µg/L USEPA drinking water standard. Also, the 10-3 resampling supported the full SWRL survey's findings on fluoride in well-water supplies. A small portion of wells (2-3%), primarily deep wells, may exceed the recommended concentration of fluoride, related to dental and skeletal fluorosis (2 and 4 mg/L). Rural well-water supplies should be tested for naturally occurring fluoride before supplements are prescribed for protection of dental cavities in young children.
Within SWRL 10-3, a pilot study was conducted to assess the use of tritium in water-quality assessment studies. Tritium is a naturally occurring isotope that can be used as a groundwater age-dating tool. From a limited number of tritium analyses, well less than or equal to >50 feet deep were several times as likely to produce groundwater with <6+/-4T.U., which averages >20 years old, than wells <50 feet deep. Groundwaters containing detectable tritium showed much higher rates of nitrate, and total- and fecal-coliform contamination, as would be expected. The data indicate a relationship between shallow wells, relatively recently recharged groundwaters, and higher rates of contaminant occurrence. This tool needs further application and testing.
The change from drought to wetter than normal conditions did not appear to affect the SWRL 10-3 water-quality results to the same degree noted in other long-term monitoring projects in the state. While atrazine detections did significantly increase in sample 10-3, the detection of other herbicides fell. Nitrate concentrations, the proportion of wells with nitrate-N>10 mg/L, and total coliform detections were largely unchanged. Fecal coliform detections did increase, but this increase was unrelated to trends in other contaminants. The state-wide nature of the SWRL 10% subset wells may be less responsive to climatic change, relative to the wells used in the longer-term monitoring projects. The long-term monitoring studies also have more frequent sampling which allows more defined trend analysis. Also, pesticides in water resources tend to show a seasonal distribution and it is not uncommon for fall or winter samples to only show detections of atrazine, typically one of the most persistent of pesticide contaminants in the environment in Iowa.