Effect of Controlled Drainage and Vegetative Buffers on Drainage Water Quality from Wastewater Irrigated Fields

Abstract

Lagoon effluent is routinely applied to agricultural fields as fertilizer to utilize nutrients derived from animal manures. In humid regions, effluent is also applied to fields in order to manage the lagoon stage within acceptable levels. To minimize the risk of applied nutrients being transported to surface waters, best management practices such as vegetative buffers and controlled drainage are often recommended. This paper presents the results of a field study to evaluate the effectiveness of controlled drainage and vegetative buffers in reducing the transport of wastewater nutrients from land application fields. Swine lagoon effluent was spray irrigated to Bermudagrass fields drained by a parallel ditch system. Drainage outflow was monitored continuously for a 3 year period. Vegetative buffers reduced the average concentration of phosphorus by about 30%, but the average concentration of nitrogen was not affected. Controlled drainage resulted in higher individual event nitrogen concentrations, which were due primarily to irrigation onto fields at times when they were too wet to be effectively irrigated. In most cases, the concentration of total N in drainage was comparable to the values observed from other agricultural fields receiving inorganic fertilizer. However, the nitrogen was mainly in the ammonium form rather than nitrate, which could pose greater environmental risk to some fish and other aquatic species in surface waters. The peak concentrations were higher for the controlled drainage treatments than for conventional drainage. The loss of nitrogen in drainage water accounted for roughly 10% of the amount applied (8% on the conventional drainage treatments and 11% on the controlled drainage treatments). On two plots (one controlled, one conventional), more than 40% of the total N transport over the 3 year study period was associated with one extended flow event when wastewater was applied several times while the water table was near the surface. These poorly scheduled irrigations onto already wet fields resulted in as much N and P transport in one event as occurred during the remainder of the study when irrigation was properly scheduled onto dry fields. These observations suggest that irrigation scheduling and proper management were more important to water quality than remedial actions such as controlled drainage or vegetative buffers.