| Popis: |
In Iowa, agricultural drainage wells (ADW9s) represent the largest deliberate transfer of degraded water into major aquifers. An estimated 700 ADW9s remove tile and surface waters from level farmland and inject it into underlying aquifers. Of 174 field-identified ADW9s, 165 were grouped into 3 clusters in north-central Iowa (17 in Floyd County, 44 in Wright County, and 104 in Humbolt-Pocahontas Counties). Geologically, most of the identified ADW9s penetrate the Des Moines glacial lobe and discharge into carbonate aquifers. These aquifers are in the intermediate ground-water flow regime, with recharge mainly from the overlying landscape and discharge mainly to rivers. A graphic flow net model indicates a progressive increase in recharge down gradient, providing increased dilution for contaminants. Modeling suggests that tile-line drainages have averaged about 10-12 ppm nitrate-N during the past decade, mostly derived from fertilizer applications. ADW effluents measured during Spring 1983 ranged between 10 and 20 ppm nitrate-N. Plumes associated with ADW9s would contain nitrate-N in concentrations approaching 20 ppm near the source, decreasing down gradient to 2-6 ppm with ground-water mixing. Joint and bedding-plane openings in the carbonate aquifer formations were measured in quarries, and the effective porosities were about 1%. If it is assumed that diffuse flow approximates isotropic and homogeneous conditions, ground-water flow velocities range from 500 to 26,000 ft/yr. Residence times for ground water in the carbonate aquifers range from a few months to a few decades, depending on the ground-water velocity and the length of individual flow paths. |
