Water, sediment, and nutrient transport from a drained topographical depression within an agricultural field in North-Central Iowa.

Abstract

Agricultural pollution is a primary source of impairment to water bodies in the US. In the agriculturally intensive Midwestern Region of the US, agricultural production is contingent upon the drainage of excess water from the landscape through constructed drainage networks. Drainage networks typically consist of horizontal subsurface tile drains and vertical surface inlets routed to drainage channels and streams. Surface inlets are utilized to drain topographical areas which often accumulate surface water. North Central Iowa is an agriculturally intensive area comprising the southeastern portion of the Prairie Pothole Region (PPR): a landscape containing a high density of enclosed topographical depressions known as prairie potholes. This region maintains a large number of surface drains in order to drain the prairie potholes and thus increase the productivity of the lands within them. Draining prairie potholes by the installation of surface drains may increase water flows as well as the loadings of agricultural water quality contaminants to receiving waters. This study focuses on the hydrology of and contaminant loading from a drained prairie pothole located in a 39.5 ha agricultural field in North Central Iowa. To assess the hydrology of the drained topographical depression, water balances were performed using climactic and water level data. Flows of water into the surface tile inlet were calculated using continuous water level data obtained via pressure transducer located near the surface drain within the topographical depression. Precipitation inflows and evaporative outflows to the pond were calculated using climactic data. Groundwater levels were monitored to assess groundwater and surface water interactions. The results of these water balances showed distinct differences in the inflows to and outflows from the depression based on antecedent conditions. In wet conditions, groundwater flow to the pond was significant source of water accounting for more than 20% of the total inflow. In drier conditions groundwater was not a source of water and infiltration comprised at least 30% of the outflow. Over the entire study period, the surface drain was the largest outflow from the pond accounting for 17160 m3, 95%, of the iii total outflow. Evaporation from and precipitation on the pond surface proved to be minor components of the water budget, accounting for 2% and 4% of the total water budget, respectively. Concentrations of potential agricultural pollutants (Total Dissolved Solids, Nitrate, Dissolved Phosphate, Total Phosphorus, Total Suspended Solids, and Dissolved Organic Carbon) were determined by the analysis of grab samples obtained from ponded water which accumulated within the depression as the result of precipitation events. Chemical loads and yields were calculated using corresponding flows into the surface drain and constituent concentrations from June until Mid-August of 2008. These values were then compared to calculated upstream loads within the receiving drainage channel to compare the relative contributions of the surface tile inlet to the watershed’s effluent. The receiving channel drains a 3640 ha catchment containing extensive drainage infrastructure immediately upstream of topographical depression. Calculated yields from the drained depression were: Total Dissolved Solids (TDS): 168 kg ha-1, Nitrate as nitrogen (NO3- -N): 8.4 kg ha-1, Orthophosphate as phosphorus (PO43- -P): 0.4 kg ha-1, Total Phosphorus as phosphorus (TP -P): 0.9 kg ha-1, Total Suspended Solids (TSS): 788 kg ha-1, and Dissolved Organic Carbon (DOC): 11 kg ha-1. Calculated yields for the receiving channel were: TDS: 722 kg ha-1, NO3- -N: 23.2 kg ha-1, PO43- -P: 0.4 kg ha-1, TP -P: 1.0 kg ha-1, TSS: 991 kg ha-1, DOC: 11 kg ha-1. These results suggest that the surface inlet has relatively low impacts on the total loads contributed receiving waters downstream.