Vance C. Kennedy

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...

Determination of the components of stormflow using water chemistry and environmental isotopes, Mattole River basin, California

The chemical and isotopic composition of rainfall and stream water was monitored during a storm in the Mattole River basin of northwestern California. About 250 mm of rain fell during 6 days (∼80% within a 42 h period) in late January, 1972, following 24 days of little or no precipitation. River discharge near Petrolia increased from 22 m3 s−1 to a maximum of 1300 m3 s−1 while chloride and silica concentrations decreased only from 3.2 to 2.1 and 11.5 to 8.6 mgl−1, respectively. Meanwhile, the isotopic composition of the river changed from δD = − 42‰, δ180 = − 6.8‰ and 40 tritium units (T.U.) to extreme values at highest flow of δD = − 35‰, δ180 = − 5.9‰ and 25 T.U. in response to volume-weighted rainfall averaging δD = − 19.5‰, δ180 = − 3.1‰ and 18 T.U. Despite much rainfall of a composition quite different from that of the prestorm river water, “buffering” processes in the watershed greatly restricted changes in the chemical and isotopic content of the river during storm runoff. Because of the physical and hydrologic characteristics of the watershed, major contributions of groundwater to stormflow are very unlikely. The large increase in dissolved chemical load observed at maximum river discharge required that extensive interaction with, and presumably penetration of, soils occurred within a few hours time. Such a large increase in chemical load also required subsurface stormflow throughout a high proportion of the watershed. Chemical and isotopic stabilization of stormflow is believed to be due mainly to displacement of prestorm soil water, with some effects on river chemistry due to rapid rain-soil interactions. The isotopic and chemical composition of prestorm soil moisture cannot readily be predicted a priori because of possible variability in rainfall composition, evaporation, and exchange with atmospheric moisture, nor can it be assumed that baseflow has a predictable relation to the chemical or isotopic composition of water displaced from soils during storms. Therefore, it seems inappropriate to draw conclusions as to the relative proportions of groundwater and rainfall in runoff from a particular storm based only on the average compositions of rainfall, stormflow, and prestorm river water, as has been done in most previous isotope hydrograph studies. Given the great variation in hydrology, topography, soil characteristics, rainfall intensity and quantity, etc. from place to place, the relative amount of overland flow, subsurface flow from the unsaturated zone and of groundwater in stormflow can vary greatly in time and space.

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.

Transport and concentration controls for chloride, strontium, potassium and lead in Uvas Creek, a small cobble-bed stream in Santa Clara County, California, U.S.A.

Abstract Three models describing solute transport of conservative ion species and another describing transport of species which adsorb linearly and reversibly on bed sediments are developed and tested. The conservative models are based on three different conceptual models of the transient storage of solute in the bed. One model assumes the bed to be a well-mixed zone with flux of solute into the bed proportional to the difference between stream concentration and bed concentration. The second model assumes solute in the bed is transported by a vertical diffusion process described by Ficks law. The third model assumes that convection occurs in a selected portion of the bed while the mechanism of the first model functions everywhere. The model for adsorbing species assumes that the bed consists of particles of uniform size with the rate of uptake controlled by an intraparticle diffusion process. All models are tested using data collected before, during and after a 24-hr. pulse injection of chloride, strontium, potassium and lead ions into Uvas Creek near Morgan Hill, California, U.S.A. All three conservative models accurately predict chloride ion concentrations in the stream. The model employing the diffusion mechanism for bed transport predicts better than the others. The adsorption model predicts both strontium and potassium ion concentrations well during the injection of the pulse but somewhat overestimates the observed concentrations after the injection ceases. The overestimation may be due to the convection of solute deep into the bed where it is retained longer than the 3-week post-injection observation period. The model, when calibrated for strontium, predicts potassium equally well when the adsorption equilibrium constant for strontium is replaced by that for potassium.

Long‐term frozen storage of stream water samples for dissolved orthophosphate, nitrate plus nitrite, and ammonia analysis

Many researchers have used freezing as an effective, short-term, water sample preservation method for subsequent nutrient analysis. In this study, filtered samples held at −16±2°C for 4–8 years were reanalyzed for orthophosphate, nitrate plus nitrite, and ammonia. Orthophosphate and ammonia concentrations decreased by 0.2 μg P/L and 5 μg N/L, respectively, at mean concentrations of 69.4 μg P/L and 246 μg N/L. Nitrate plus nitrite increased by 1.1 μg N/L at a mean concentration of 139.1 μg N/L. An anaerobic well sample proved to be unsuitable for freezing because it lost significant amounts of orthophosphate during the freezing process. None of the differences observed over long periods of frozen storage were more than twice the estimated standard deviation of the analytical methods used in the study. The small changes observed demonstrate the effectiveness of frozen storage as a means of nutrient preservation in water samples that are unaffected by the freezing process itself.

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.

Interparticle migration of metal cations in stream sediments as a factor in toxics transport

Sorption of metal cations by stream sediments is an important process affecting the movement of released contaminants in the environment. The ability of cations to desorb from one sediment particle and subsequently sorb to another can greatly affect metal transport rates but rates for this process have not been reported. The objective of this study was to determine the rate at which sorbed metals can migrate from contaminated sediment particles to uncontaminated sediment particles as a function of the concentration of the contaminating solution and the duration of the contact with the contaminating solution. Samples of small sediment particles were exposed to solutions containing cobalt, after which they were rinsed and combined with larger uncontaminated sediment particles in the presence of stream water. Initial concentrations of the contaminating solution ranged from 1ng/l to 1000mg/l and exposures to the contaminating solution ranged from 6h to 14 days. The rate of the migration increased with increasing concentrations in the contaminating solution and with decreasing times of exposure to the contaminating solution. Under the conditions of these experiments, the time required for the migration to reach equilibrium was on the order of months or longer. In separate experiments, the kinetics of adsorption and desorption of cobalt were measured as a function of concentration of the contaminating solution. The time required to reach adsorption equilibrium increased with increasing concentration in the contaminating solution. Times to sorption equilibrium were on the order of months. Desorption was much slower than adsorption and, together with intraparticle diffusion, probably controls the rate of migration from contaminated to uncontaminated sediment. The results of this study show that interparticle migration of metal cations can proceed at significant rates that are strongly influenced by the length of time that the metal has been in contact with the sediment.