Characterization of hyporheic zone processes of a northern prairie stream

A hyporheic zone occurs beneath and adjacent to streams where surface water and groundwater mix. This zone is known to be a reservoir for solutes and a habitat for interstitial organisms. Hyporheic zone boundaries in the Tongue River, North Dakota, were investigated through examination of the physicochemical and biological gradients present in the stream channel and bank sediments. The effects of cattle grazing at the streamside interface of the riparian zone were also examined at two locations to determine land use impacts on the hyporheic zone. Electrical conductivity (EC) and ammonium gradients were observed beneath the stream channel via nests of mini-piezometers (1.27-cm clear polyethylene tube) and wells (2.54-cm PVC). EC and ammonium increased immediately below the stream, then decreased laterally and with depth. Discontinuities in the EC gradient may indicate where metabolic or redox reactions occur under the streambed due to mixing of surface water and groundwater, or conditions in the sediments. Discontinuities in the ammonium gradient may represent a boundary between dissimilatory nitrate reduction to ammonium and a nitrification-denitrification couple, as well as subtle changes in hydraulic gradient. Discontinuities in gradients and zones beneath the stream channel likely fluctuate as subsurface processes shift temporally. Changes in biotic activity could have caused ammonium concentrations in sub-channel water to decrease from spring to summer and then increase from summer to fall at both sites. Chemical and biological gradients in the Tongue River differ from other studies presumably because of regional differences in geology, climate, and hydrology. The density and diversity of interstitial meiofauna populations may provide information about the general physical and chemical conditions of streambed sediments. Meiofauna diversity and population size decreased with ·depth at both study sites. High levels of ammonium and/or ambient reduced conditions in zones beneath the channel could exclude most meiofauna species. Meiofauna population dynamics also may indicate stream reach health. Cattle grazing in the riparian zone appeared to affect chemical and biological gradients. Ammonium concentrations and EC were elevated at the grazed site (\u3e10 mg/Land 1300 µSiem, respectively). The lack of riparian vegetation and direct input of cattle waste may have caused higher EC and ammonium at the stream margin of the grazed site. Less dense and diverse populations of meiofauna at the grazed site were likely caused by greater disturbance of the sediments, lack of woody debris and preferred substrate, and more reduced conditions in the channel sediments. Differences in ammonium concentrations in subchannel water between the sites were greater than differences in nitrate concentrations in surface water. This implies that sampling designed to evaluate stream health must consider all sources and reservoirs of pollutants, and cannot solely depend on surface water analysis