IRRIGATION AS A CONSERVATION PRACTICE TO REDUCE NITRATE CONCENTRATIONS IN COASTAL PLAIN GROUNDWATER, EASTERN SHORE CHESAPEAKE BAY WATERSHED

The Eastern Shore of the Chesapeake Bay makes up 7 percent of the bay watershed, but contributes disproportionately large loads of nitrogen that contribute to the ecological and economic degradation of the bay. Of the total amount of nitrogen reaching the bay from the Eastern Shore over 90 percent is from agricultural sources and over 70 percent is nitrate from groundwater. Study of groundwater is important as changes in water quality associated with changes in agricultural conservation practices will be seen first in shallow groundwater directly beneath fields but it will take years to decades for these changes to be seen in local streams. Groundwater chemistry, however, is rarely included in studies of crop production and nitrogen utilization in this area. Conservation practices that hold nitrogen in the soil zone and increase nitrogen utilization by crops should limit the amount of nitrate available to leach into groundwater and eventually discharge to streams. Irrigation is increasing rapidly on the Eastern Shore to increase crop production in response to growing markets for corn, soybeans, and small grains. Use of irrigation is hypothesized to increase nitrogen uptake and leave less residual nitrogen in the soil that could potentially convert to nitrate and leach into groundwater than dryland farming. Two study sites on farms with similar crops on adjacent fields—one with center-pivot irrigation and one with dryland farming—but different soil types and surficial aquifer conditions are being studied to test this hypothesis. Each site is instrumented with a network of wells and suction lysimeters. Data on crop production and nitrogen uptake, cover crops, total nitrogen in soils, soil water and groundwater chemistry, soil moisture, groundwater levels, and precipitation are being collected to develop a mass balance of nitrogen inputs and outputs to each field over multiple growing seasons and to observe changes in groundwater quality over time. Initial data from the 2014 corn crop on one of the study farms showed a greater nitrogen use efficiency and less residual nitrogen in the irrigated field than the dryland field. This result seems to support the hypothesis that irrigation could help to reduce nitrate concentrations in groundwater in this hydrogeologic setting.