MAGNITUDE OF NITRATE REMOVAL IN STREAMBED SEDIMENTS AND THE EFFECT OF PORE-WATER VELOCITY

Recent studies in the coastal plain of Virginia and North Carolina have quantified nitrate loss as groundwater travels through sandy sediments directly underlying the stream channel of gaining streams. The magnitude of nitrate removal in the streambed sediments of 11 low-order streams on the Virginia Eastern Shore (VAES) that drain agricultural land and flow to the Atlantic Ocean was examined to evaluate the hydrological control on extent of nitrate removal by developing correlations with pore-water flow velocity. A portable oil-water manometer drive point was constructed, similar to the design of Kennedy et al. (2007), in order to measure the relative hydraulic head gradient between surface water and groundwater as well as to collect pore-water samples for chemical analysis.

The drive point device was used to sample streams in both longitudinal and latitudinal directions, and was inserted into streambed sediments from 10 – 60 cm depths. The hydraulic head gradients ranged from -0.108 – 0.164 relative to the overlying stream, and of the 25 gradient measurements collected at ~60 cm depth in the center channel of 7 streams, 72% showed an upward gradient. In general, hydraulic gradient profiles suggest a correlation of decreasing gradient with decreasing depth below the streambed. Dissolved oxygen concentrations of pore water ranged from 0.4 – 7.3 mg/L with the majority of samples <3 mg/L, sufficiently depleted to allow for denitrifying bacteria to be active. Indirect indication of denitrification is provided by measurements of nitrate concentrations in pore water. One vertical profile showed a decrease from 7 mg N/L at 60 cm depth below the streambed to <1 mg N/L at 10 cm depth, greatly reducing the nitrate load carried by the stream. By quantifying the amount of nitrate removal in the streambed sediments for multiple streams on the VAES, we gain insights to the extent and variability of the amelioration of groundwater contaminated by nitrogen-based fertilizers.