Closure of Agricultural Drainage Wells (ADWs) Improves Water Quality

Many of Iowa's rich agricultural soils, particularly those in north-central Iowa, are poorly drained and at times contain excess water that can interfere with field operations or even ruin crops. In these areas, farm fields are often artificially drained by buried tiles leading to drainage ditches or to streams. A less commonly used drainage method is the agricultural drainage well (ADW), a drilled shaft that funnels drainage water into the underlying bedrock (see illustration, left).

The upper parts of ADWs are often cistern-like structures that are the discharge points 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 limestone aquifers, which can accept large quantities of drainage water without clogging. These aquifers are also excellent sources of groundwater for domestic, industrial, and municipal water supplies.

An estimated 292 ADWs currently exist in Iowa, and nearly all were constructed between 1900 and 1950. From a statewide perspective, these wells are relatively uncommon. However, over 90% of the known ADWs in Iowa are concentrated in four counties: Floyd, Humboldt, Pocahontas, and Wright.

It has long been recognized that ADWs pose a threat to groundwater quality in Iowa, as they provide a direct route for delivery of contaminants such as nitrate, pesticides, bacteria, and sediment to the underlying bedrock aquifers. The impacts of ADWs are most noticeable after a rainfall, when surface water and tile drainage are delivered to the groundwater via the ADWs. During extended dry periods, drainage inputs are minimal, and ADW impacts are less severe.

Some ADWs are connected to drainage systems that accept water from road ditches. Spills or leaks of harmful substances into these ditches could quickly and directly impact groundwater supplies. A more recent concern is the placement of large-scale animal confinement facilities in close proximity to ADWs. The land application of manure from these facilities or a spill from a manure storage structure to a nearby ADW poses an additional threat to groundwater quality.

Past Geological Survey Bureau (GSB) groundwater-quality studies in Floyd County have shown that ADWs delivered agricultural contaminants, specifically nitrate and commonly used pesticides, into the Devonian-age limestone aquifers, otherwise protected by about 30 feet of glacial till (clay-rich sediment). Since 1984, GSB has monitored four bedrock wells (ranging in depth from 103 to 360 feet) near several ADWs in Floyd County. Results showed elevated nitrate concentrations and pesticide detections in the wells (see graphs, above).

In 1994, the Floyd County Soil and Water Conservation District inventoried ADWs in the county. Owners of ADWs were asked about their willingness to close these wells and develop alternate surface outlets for the tile water. A total of 23 ADWs were voluntarily closed in Floyd County, including several that were more than 300 feet deep. In December 1994, three ADWs (two 65 feet deep; one over 300 feet deep) located within 3,000 feet of the four monitored bedrock wells mentioned above were closed, and tile water was diverted to a surface outlet.

Since closure of these ADWs, water quality has improved in all four bedrock wells. Nitrate concentrations have declined, and the number and concentration of pesticides detected also have declined (see graphs, above). Although the nitrate concentration in Well #1 has declined since the ADWs were closed, it remains slightly above the U.S. Environmental Protection Agency's established drinking water standard of 45 milligrams per liter (mg/L). The continued elevated nitrate concentrations and high frequency of pesticide detection indicate that this well also is impacted by contaminants entering the bedrock by a pathway other than the ADWs. Located within a few miles of this bedrock well are areas where bedrock lies close to the land surface. These areas of shallow bedrock are very susceptible to contaminants entering the groundwater through infiltration and are likely contributing nitrate and pesticides to the aquifer in which the shallowest bedrock well (#1) is located.

Water quality has also improved in a nearby private well that serves as a source of drinking water. Prior to closure of the ADWs, nitrate concentrations in this well were greater than 65 mg/L. Since closure, concentrations are less than 20 mg/L.

Additional ADW closures will occur in Iowa, as a 1997 Iowa law requires that all ADWs located within a surface drainage area that includes a state-permitted earthen manure storage structure for an animal confinement facility must be closed by December 31, 1999 (later extended to December 31, 2001). Will closure of ADWs in other locations result in improved groundwater quality? Based on results from this monitoring project, the answer is probably yes.

This water-quality monitoring project was funded by the Iowa Department of Agriculture and Land Stewardship.

Adapted from Iowa Geology 1999, Iowa Department of Natural Resources