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 4 7% 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 <24 h for wells <5 m from the wetted channel. Coinjected nitrate was not conservative. Two wells were significantly (P < .05) higher in nitrate-N than would be predicted from chloride, while four were significantly lower. Wells 2.0-4.0 m from the wetted channel tended to have higher nitrate concentration than predicted, whereas nitrate sink locations tended to have transport distances >4.3 m. The capacity of the hyporheic zone for transient solute storage and as potential biological habitat varies with channel morphology, bed roughness, and permeability. A conceptual model that considers the groundwater-stream water interface as the fluvial boundary is proposed. Emerging paradigms of the riverine network should consider the hyporheic zone and associated nutrient cycling as an integral component of fluvial structure and function.

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, die! 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 I 0-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 stream's 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 NOr N was II 7 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.

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Fine-grained particulate material can pass through a 0.45-#m membrane filter and cause major errors up to an order of magnitude or more in the determination of AI, Fe, Mn, a•d Ti dissolved in natural waters. Other elements enriched in clay-size sediments but normally very low in solution concentration may also be affected. Membrane filters have come into widespread use in recentyears for the filtration of water samples prior to analysis.Although filters of various pore sizes are available, membranes with 0.45-urn nominal pore size have been most commonly used. However, clay minerals of the type found in stream sediments can be much smaller than 0.45 #m [Kennedy, 1965;Grim, 1968], and such materials may pass the filter in sufficient quantity to influence significantly the reported concentration of some elements in solution. The constituents most affected are AI, Fe, Ti, Mn, and those asmciated trace elements that are concentrated in sparingly soluble oxyhydroxides and clay minerals. Erroneous values for concentrations of these elements in true solution are of special concern when mineralogic controls on water chemistry are being evaluated.When filtered water samples are analyzed by spectrographic or neutron activation methods, dissolved constituents and suspended particulates are determined simultaneously in a composite, and all elements in suspended material will be reported as in solution. Many wet chemical techniques determine only dissolved constituents, but it is common practice to acidify filtered samples to prevent precipitation or adsorptive losses, and this acid could be expected to attack any dispersed fine particulates in suspension, thereby yielding added dissolved constituents.The purpose of this study was to obtain more information on the extent to which material passing through filters might be affecting the reported concentrations of dissolved constituents in stream water. This was done by passing separate aliquots of various water samples through filters of different pore sizes and analyzing the resultant tiltrates chemically or, in some instances, spectrographically. Detailed evaluation of data obtained by Durum and Haffty [1961] using 2-•tm filters was included with the information gained in the present study. PREVIOUS WORKThere has been appreciation of the effect of filters on the results of analyses of trace constituents for more than 25 years, but much of the concern has been with chemical interaction between the filter and solutions being filtered rather than with the possible effects of any solids passing the filter, as long as the tiltrate appeared clear. Sandell [1944] called atten-Copyright (D !974 by the American Geophysical Union. tion to the fact that filter paper may sorb such metals as Pb and Cu from solution and recommended that inorganic filter media be used. Marvin et al. [ ! 970] found that several types of filters are contaminated with Cu, which can be partly removed or added to depending upon the ch.aracter of the sample being filtered. Others [Robertson, 1968; Spencer and Ma...

An experimental injection was performed to study the transport of stream water solutes under conditions of significant interaction with streambed sediments in a mountain pool‐and‐riffle stream. Experiments were conducted in Little Lost Man Creek, Humboldt County, California, in a period of low flow duringwhich only a part of the bank‐full channel held active surface flow. The injection of chloride and several trace cations lasted 20 days. In this report we discuss the results of the first 24 hours of the injection and survey the results of the first 10 days. Solute‐streambed interactions of two types were observed. First, the physical transport of the conservative tracer, chloride, was affected by intergravel flow and stagnant watt, zones created by the bed relief. Second, the transport of the cations (strontium, potassium, and lithium) was appreciably modified by sorption onto streambed sediment. In the stream the readily observable consequence of the solute‐streambed interactions was an attenuation of the dissolved concentration of each of the tracers. The attenuation in the stream channel occurred concurrently with the storage of tracers in the streambed via both physical and chemical processes. All tracers were subsequently present in shallow wells dug several meters from the wetted part of the channel. Sediment samples collected approximately 3 weeks after the start of the injection contained increased concentrations of the injected cations.

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A significant fraction of rhodamine WT dye was lost during a short term multitracer injection experiment in a mountain stream environment. The conservative anion chloride and the sorbing cation lithium were concurrently injected. In‐stream rhodamine WT concentrations were as low as 45 percent of that expected, based on chloride data. Concentration data were available from shallow‘wells’dug near the stream course and from a seep of suspected return flow. Both rhodamine WT dye and lithium were nonconservative with respect to the conservative chloride, with rhodamine WT dye closely following the behavior of the sorbing lithium. Nonsorption and sorption mechanisms for rhodamine WT loss in a mountain stream were evaluated in laboratory experiments. Experiments evaluating nonsorption losses indicated minimal losses by such mechanisms. Laboratory experiments using sand and gravel size streambed sediments show an appreciable capacity for rhodamine WT sorption. The detection of tracers in the shallow wells and seep indicates interaction between the stream and the flow in the surrounding subsurface, intergravel water, system. The injected tracers had ample opportunity for intimate contact with materials shown in the laboratory experiments to be potentially sorptive. It is suggested that in the study stream system, interaction with streambed gravel was a significant mechanism for the attenuation of rhodamine WT dye (relative to chloride).