NATURAL NITRATE REMOVAL IN SHALLOW SUBSURFACE STREAM FLOWS

As agricultural growth increases across the planet, more anthropogenic nitrate from fertilizers and sewage effluent is contributed to the aquatic system, exacerbating both ecosystem- and human-health issues. Nitrate is naturally processed and removed within the environment, and those processes have been observed in a segment of substrata and porewater below streams called the hyporheic zone (HZ). The interaction of stream water with groundwater can promote denitrification; however, the rate of nitrate reduction within the HZ is unknown. This study determined the extent of surface water-groundwater interactions in a HZ and assessed the nitrate reduction in this zone via monthly sampling of three wells inserted along the length of T3, a stream located in the agriculturally dominated, Central Illinois landscape. Samples were taken from 10, 20, 30, and 50 cm below the streambed, the stream, and a groundwater well from spring to fall of one year to assess the full mixing patterns and nitrate contributions of the landscape to the stream system. The chemical composition of the stream water, groundwater, and HZ waters were analyzed using an Ion Chromatograph and applied in a mixing-model.

Results show that stream water and groundwater contribute proportionally inverting amounts to water flow through the depth of the HZ. The conservative ion chloride is a chemical indicator of mixing in waters, and in the studied HZ, chloride concentrations were 48.8% higher in surface water than groundwater, and a gradient of change between these two endmembers was observed along depth throughout the HZ. Reducing nitrate levels along depth can be positively correlated to this gradient of mixing in the HZ. This relationship supports that the mixing of surface water and groundwater that occurs along the depth of the HZ dilutes the surface water and removes its excess nitrate. A better understanding of how different water sources contribute to the HZ and how that water flows through this zone will better equip regulators and remediators to use streams and their hyporheic zones to remove excess nitrate from agricultural runoff, contributing to healthier ecosystems and drinking water.