Gary W. Zellweger

RETENTION AND TRANSPORT OF NUTRIENTS IN A THIRD-ORDER STREAM IN NORTHWESTERN CALIFORNIA: HYPORHEIC PROCESSES'

Chloride and nitrate were coinjected into the surface waters of a third—order stream for 20 d to examine solute retention, and the fate of nitrate during subsurface transport. A series of wells (shallow pits) 0.5—10 m from the adjacent channel were sampled to estimate the lateral interflow of water. Two subsurface return flows beneath the wetted channel were also examined. The conservative tracer (chloride) was hydrologically transported to all wells. Stream water was >88% of flow in wells <4 m from the wetted channel. The lowest percentage of stream water was 47% at a well 10 m perpendicular to the stream. Retention of solutes was greater in the hyporheic zone than in the channel under summer low—flow conditions. Nominal travel time (the interval required for chloride concentration to reach 50% of the plateau concentration) was variable by well location, indicating different flow paths and presumably permeability differences in subsurface gravels. Nominal travel time was M 24 h for wells <5 m from the we...

Influence of diurnal variations in stream temperature on streamflow loss and groundwater recharge

We demonstrate that for losing reaches with significant diurnal variations in stream temperature, the effect of stream temperature on streambed seepage is a major factor contributing to reduced afternoon streamflows. An explanation is based on the effect of stream temperature on the hydraulic conductivity of the streambed, which can be expected to double in the 0° to 25°C temperature range. Results are presented for field experiments in which stream discharge and temperature were continuously measured for several days over losing reaches at St. Kevin Gulch, Colorado, and Tijeras Arroyo, New Mexico. At St. Kevin Gulch in July 1991, the diurnal stream temperature in the 160-m study reach ranged from about 4° to 18°C, discharges ranged from 10 to 18 L/s, and streamflow loss in the study reach ranged from 2.7 to 3.7 L/s. On the basis of measured stream temperature variations, the predicted change in conductivity was about 38%; the measured change in stream loss was about 26%, suggesting that streambed temperature varied less than the stream temperature. At Tijeras Arroyo in May 1992, diurnal stream temperature in the 655-m study reach ranged from about 10° to 25°C and discharge ranged from 25 to 55 L/s. Streamflow loss was converted to infiltration rates by factoring in the changing stream reach surface area and streamflow losses due to evaporation rates as measured in a hemispherical evaporation chamber. Infiltration rates ranged from about 0.7 to 2.0 m/d, depending on time and location. Based on measured stream temperature variations, the predicted change in conductivity was 29%; the measured change in infiltration was also about 27%. This suggests that high infiltration rates cause rapid convection of heat to the streambed. Evapotranspiration losses were estimated for the reach and adjacent flood plain within the arroyo. On the basis of these estimates, only about 5% of flow loss was consumed via stream evaporation and stream-side evapotranspiration, indicating that 95% of the loss within the study reach represented groundwater recharge.

Retention and Transport of Nutrients in a Third‐Order Stream: Channel Processes

Chloride was injected as a conservative tracer with nitrate to examine nitrate retention (storage plus biotic uptake) and transport in a 327-m reach of a third-order stream draining a forested basin in northwestern California. Prior to injections, diel patterns of nutrient concentrations were measured under background conditions. Nitrate concentration of stream water increased downstream, indicating that the reach was a source of dissolved inorganic nitrogen to downstream communities under background, low-flow conditions, despite uptake by photoautotrophs. At the onset of continuous solute injection over a 10-d period, timing the passage of the solute front indicated that storage dominated nitrate retention. Instantaneous concentration differences at the base of the reach at hour 24 indicated that biotic uptake accounted for 13% of the nitrate amendment while hydrologic storage constituted 29%. Corrected for groundwater dilution (11.7%), saturation of the streams channel and hyporheic zones was not complete until 6.8 d of continuous injection. By day 3 nitrate retention was dominated by biotic processes. Biotic uptake was greatest during daylight hours indicating retention by photoautotrophs, but also occurred during darkness. After 10 d of continuous injection, mass balance calculations indicated that 29% of N (339 g) was retained from the total injected (1155 g), while the balance of injected nitrate was transported downstream. Storage of NO3-N was 117 g or 10% while biotic uptake was 222 g or 19%. Periphyton biomass on slides, chlorophyll a both on slides and on natural cobbles, and net community primary production all indicated a lag in periphyton response to nitrate amendment. Earliest indicators of a biotic response to nutrient amendment were decreases in both tissue C/N and epilithic respiration.

In situ retention-transport response to nitrate loading and storm discharge in a third-order stream

Nitrate retention was assayed in a 264-m reach of a third-order stream, Little Lost Man Creek, Humboldt County, California, USA. Nitrate budgets (24-48 hours) were calculated under background conditions, and during four other intervals of modified nitrate concentration caused by nutrient amendment or storm-enhanced discharge. Under background, low-flow conditions, the reach was a source of nitrate to downstream communities. Retention during the first 36 hours of nitrate amendment was dominated by storage in the hyporheic zone and later by biotic uptake as storage zones became saturated (plateau concentration). The increase in net retention caused by increased nitrate concentration decreased output/input (O/I) ratio from 1.11 before amendment to 0.61 after 36 hours, and to 0.86 after transient storage zones were filled. Dilution, caused by a nearly four-fold increase in discharge, increased biotic retention and also export as previously stored nitrate leached from the hyporheic zone into the channel. Nitrate continued to leach from the hyporheic zone seven days after the amendment ended. This type of response may enhance biotic nutrient cycling by providing waters of higher nutrient concentration to partially scoured epilithic surfaces following reset of the benthic community by a major storm.

Humic substances and trace metals associated with Fe and Al oxides deposited in an acidic mountain stream

Hydrous iron and aluminum oxides are deposited on the streambed in the confluence of the Snake River and Deer Creek, two streams in the Colorado Rocky Mountains. The Snake River is acidic and has high concentrations of dissolved Fe and Al. These metals precipitate at the confluence with the pristine, neutral pH, Deer Creek because of the greater pH (4.5–6.0) in the confluence. The composition of the deposited oxides changes consistently with distance downstream, with the most upstream oxide samples having the greatest Fe and organic carbon content. Fulvic acid accounts for most of the organic content of the oxides. Results indicate that streambed oxides in the confluence are not saturated with respect to their capacity to sorb dissolved humic substances from streamwater. The contents of several trace metals (Mn, Zn, Cu, Pb, Ni and Co) also decrease with distance downstream and are correlated with both the Fe and organic carbon contents. Strong metal-binding sites associated with the sorbed fulvic acid are more than sufficient to account for the trace metal content of the oxides. Complexation of trace metals by sorbed fulvic acid may explain the observed downstream decrease in trace metal content.