Groundwater Quality Response to Closure of Agricultural Drainage Wells in Floyd County, Iowa

Figure 1. An agricultural drainage well (ADW) is a drilled shaft that funnels excess drainage water into the underlying bedrock. The upper parts of the ADWs are often cistern-like structures that form the discharge point for one or more tile drainage lines; some wells are also designed to take surface runoff. ADWs are generally 5 to 10 inches in diameter and are cased into the underlying bedrock. Virtually all ADWs in Iowa discharge into fractured carbonate aquifers, as these strata can accept large quantities of drainage water without clogging. These aquifers are also excellent sources of groundwater for domestic, industrial, and municipal water supplies.

Figure 2. The above map shows the distribution, by county, of known ADWs in Iowa. The number of ADWs in Iowa has never been precisely determined. In 1997, an updated inventory of ADWs in Iowa was completed, and 292 active ADWs were identified. From a statewide perspective, these wells are relatively minor features. However, over 90% of the known ADWs are concentrated in four counties: Floyd, Humboldt, Pocahontas, and Wright counties.

Iowa is not the only state with ADWs. Other states include, but are not limited to, Idaho, New York, Texas, Arizona, Minnesota, Ohio, Georgia, Wisconsin, North Carolina, North Dakota, Oregon, Washington, and Michigan.

Figure 3. Four geologic regions, representing different hydrogeologic settings, occur in Floyd County.

The Deep Bedrock region is an area where the carbonate bedrock is buried by greater than 50 feet of Quaternary (glacially deposited) surficial material. These regions are relatively "protected" from surficially derived contaminants. An exception is the Deep Bedrock region in central Floyd County. In this area, the presence of ADWs allows contaminants to bypass the protective surficial materials and directly enter the underlying bedrock aquifers.

Most of Floyd County is in the Shallow Bedrock region, an area characterized by 50 feet or less of Quaternary deposits overlying bedrock, and an area where few sinkholes are found. Groundwater in the Shallow Bedrock region is susceptible to infiltration, particularly in areas where the Quaternary deposits are less than 50 feet.

The Karst region contains very shallow bedrock, generally at a depth of less than 25 feet, where numerous open sinkholes occur. In the Karst region, groundwater reflects the impacts of both infiltration and direct run-in of surface water.

The Incipient Karst region is an area of very shallow bedrock, generally less than 20 feet with numerous incipient sinkholes (sinkholes that are beginning to form) on broad, low-relief upland divide areas. It is marked by high rates of infiltration recharge and little, if any, surface water run-in recharge.

Figure 4. In December 1994, the three drainage wells (ADW-1, ADW-2, and ADW-3) nearest the bedrock piezometer nest were closed. The tile lines previously draining to these three ADWs were connected and the tile water diverted by a single tile line (ADW-Drain) into a constructed drainage ditch, along which the water travels for approximately three miles before emptying into Beaver Creek. Two additional ADWs in close proximity to the monitored sites were closed the summer of 1995.

Water-quality monitoring occurred at piezometer nest FM3, five nearby private wells, ADW-Drain, and two surface water sites.

Figure 5. Stratigraphic and hydrogeologic studies suggest that the Devonian strata in Floyd County comprise a three-part aquifer system with the major water-producing carbonate strata separated by intervening shales and shaley carbonates. These aquifers are referred to as the upper, middle, and lower aquifers.

ADW-1 discharged water into all three aquifers while ADW-2 and ADW-3 discharged water into the upper aquifer. All three ADWs were closed in December 1994.

The five monitored private wells were completed in the upper aquifer.

MCL = Maximum Contaminant Level;
ND = not detected.

Figure 6. Monitoring of the tile water entering the ADWs showed that while ADWs negatively impact groundwater within one to two miles of several ADWs, not all wells within this distance show ADW impacts, and the impacts vary with time. ADW impacts were most noticeable following runoff and/or infiltration generating conditions, when surface and/or tile drainage is delivered to the groundwater via ADWs. ADW impacts are less, or not noticeable during extended dry periods when drainage inputs are insignificant.

Nitrate-N concentrations in the ADWs ranged from 2.0 to 35.0 mg/L. Alachlor, atrazine, cyanazine, deethylatrazine (a metabolite or breakdown product of atrazine), metolachlor, and metribuzin were detected at the three ADWs at concentrations ranging from 0.11 to 5.90 �g/L. Atrazine, cyanazine, and deethylatrazine were the most frequently detected pesticides.

		MCL = Maximum Contaminant Level; ND = not detected.

Figure 7. FM3-1 is located in the upper part of the upper aquifer. Since closure of the ADWs, nitrate-N concentrations have declined and the frequency of atrazine detection has increased. The range in nitrate-N concentrations has declined from the pre- to post-closure period. With the removal of the point source inputs of ADWs, the more consistent, yet elevated, nitrate-N concentrations likely reflect nonpoint source inputs to the upper aquifer from nearby, geologically sensitive areas.

		MCL = Maximum Contaminant Level; ND = not detected.

Figure 8. FM3-4 is located in the lower aquifer. Nitrate-N concentrations declined from the pre- to post-closure periods. The frequencies of pesticide detection also declined. Since closure, there has only been one detection of one pesticide. This detection (metolachlor; 0.10 �g/L) occurred shortly after the ADWs were closed.

MCL = Maximum Contaminant Level.

Figure 9. Nitrate-N concentrations declined from the pre- to post-closure period for all piezometers except the glacial till piezometer (FM3-T). This piezometer was not impacted by the ADWs. The decline in nitrate-N concentrations was statistically significant for FM3-1, FM3-2, and FM3-4.

Figure 10. The frequency of atrazine detections declined in all wells except FM3-T and FM3-1. Although the atrazine frequency increased at FM3-1, the mean atrazine concentration declined.

Mean atrazine concentrations are located above each bar on the graph.

MCL = Maximum Contaminant Level.

Figure 11. Of the private wells monitored, C-Well showed the most significant decline in nitrate-N concentrations. It is the nearest private well to the closed ADWs. Since closure of the ADWs, nitrate-N concentrations at C-Well declined from >15 mg/L to concentrations at or below 5 mg/L during most months. The decline in nitrate-N concentrations at this site coincides with closure of the three ADWs.

The four other private wells have not shown a similar response to C-Well, suggesting that those wells were not previously impacted by the ADWs that were closed.

Table 1. Nitrate-N concentrations in the glacial till and bedrock piezometers (site FM3) in Floyd County, both prior to and after closure of nearby ADWs.

Table 2. Frequency of pesticides detected, range in concentration, and mean and median concentrations for pesticides in the glacial till and bedrock piezometers (site FM3) in Floyd County, both prior to and after closure of nearby ADWs.

Acknowledgements

The Floyd County Agricultural Drainage Well Project has been supported, in part, by Water Quality Protection Funds administered by the Iowa Department of Agriculture and Land Stewardship, Division of Soil Conservation via the Floyd County Soil and Water Conservation District, and through analytical services of The University of Iowa Hygienic Laboratory.

Presented as a poster at the From the Heartland to the World: The Importance of Clean Water Conference, March 24-26, 1999, Ames, IA.