Garey A. Fox

Unsaturated hyporheic zone flow in stream/aquifer conjunctive systems

Abstract Saturated flow is typically assumed for seepage from a stream underlain by an alluvial aquifer. However, if the water table falls a sufficient distance below a semipervious streambed, the head losses in this less conductive layer will cause the region beneath the stream, or hyporheic zone, to become unsaturated. Hyporheic zone flow is defined loosely in this research as the flow that occurs underneath the streambed. Unsaturated flow transforms streams from constant head boundaries to constant flux boundaries, impacting the biogeochemistry in the hyporheic zone. The objective of this paper is to discuss the development and implications of unsaturated flow beneath the streambed. Conditions under which saturated or unsaturated flow occurs and the characteristics of each flow regime are discussed. Next, the effect of unsaturated flow is illustrated for the case of stream leakage induced by a well pumping from an aquifer that is hydraulically interacting with a partially penetrating stream. Prior analytical solutions for alluvial well depletions fail to model unsaturated flow between the streambed and water table. An approximating solution is proposed to estimate aquifer drawdown and stream depletion under saturated/unsaturated hyporheic zone flow conditions.

Organic Carbon Concentrations in Hyporheic Zone Sediments: A Tool for Measuring Stream Integrity

Effects of channel incision on sand-bed stream carbon reservoirs were examined. Channel incision may deplete hyporheic zone C stores due to bed erosion, less frequent hydrologic exchanges between stream and floodplain, and paucity of riparian vegetation and large woody debris. Presented are organic C concentrations found in hyporheic sediments before and after an incised stream rehabilitation project and in three adjacent streams in northern Mississippi. The sampled streams comprise a spectrum of physical conditions corresponding to the conceptual channel evolution model (CEM). Carbon concentrations in the upper 10 cm of the bed ranged from 0.24 + 0.36% for a nonincised reference site to only 0.01 + 0.02% for aggradational incised channels. Carbon concentrations generally declined with increase in stage of the CEM, increased with increasing percent canopy over the study reach and were not directly related to large woody debris (LWD) density. These findings suggest factors linking ecological degradation to channel incision and prospective pathways for stream rehabilitation design.

Hyporheic and Total Storage Exchange in Small Sand-Bed Streams

Nutrient processing and carbon storage in stream ecosystems are linked to hydraulic retention. Hydraulic retention refers to the departure of stream flow from ideal “plug flow,” and reflects fluid movement through surface and hyporheic storage zones. Most existing information about hyporheic exchange is based on flume studies or field measurements in relatively steep streams with beds coarser than sand. Stream tracer studies may be used to quantify overall hydraulic retention, but disaggregation of surface and hyporheic retention components remains difficult. A stream tracer approach was used to compute the rates at which stream water is exchanged for water in storage zones (total storage) in short reaches of two small, sand-bed streams. Tracer curves were fit to the one-dimensional transport with inflow storage (OTIS-P) model. Networks of minipiezometers were used to measure hyporheic exchange. The rate of exchange between main channel flow and surface storage zones was assumed equal to the difference between total storage exchange computed by the modeled tracer curves and hyporheic exchange. This technique was used to measure the effects of flow obstructions (large wood) and differences in bed morphology and grain size on surface and hyporheic exchange. Parameters describing total retention were in the upper 50% of data compilations published by others that represent a range of stream sizes and morphologies, but hyporheic exchange was only 0.01% to 0.49% of total exchange. Retention did not correlate to differences in median bed material size, but increased with flow obstruction. Hyporheic storage was an insignificant component of total hydraulic retention in the small sand-bed streams studied.

Earthworms and E. coli: A Perilous Combination for Drainflow Water Quality

Researchers at Oklahoma State University (OSU), Iowa State University (ISU), and the USDA-ARS National Soil Tilth Laboratory (NSTL) have formed a unique research team to solve an important water quality problem. This team is addressing the transport of pathogens (specifically, the indicator organism Escherichia coli or E. coli) through soils and, more specifically, the role of macropores in the transport of E. coli to subsurface drains. Pathogen contamination of water supplies is now considered one of the top water-quality issues in the United States and around the world.

An Improved Express Fraction for Modeling Macropore/Subsurface Drain Interconnectivity

The rapid transport of contaminants through macropores and into subsurface drains is a concern. Recent research has proposed methods for incorporating this direct connectivity into contaminant transport models. For example, the one-dimensional pesticide fate and transport model, Root Zone Water Quality Model (RZWQM), was modified to include an express fraction parameter based on the percentage of macropores in direct hydraulic connection to subsurface drains. When macropore flow first reached the top of the water table (point midway between the drains), a macropore express fraction of water and chemical was routed directly into the subsurface drain, which improved predictions of concentration peaks. The remaining water and chemical was allowed to fill and mix with the water table, resulting in a concentration bulge at the water table. This research proposes an updated express fraction for RZWQM, which distributes water across all saturated layers between the drain and water table. Implicitly assumed is a uniform spatial distribution of macropores. This updated express fraction is evaluated using data from two isoxaflutole/metabolite field experiments in Allen County and Owen County IN (2000), where concentrations of parent and metabolite were measured in the drain flow. The results showed a slight improvement in the prediction of chemical concentrations on the recession limbs of drainage hydrographs.

Unsaturated Hyporheic Zone Flow in Analytical Models for Stream/Aquifer Interaction

Pumping from groundwater resources that are hydraulically interactive with adjacent nstreams induces a water flux from the stream to the aquifer. Analytical models of stream/aquifer nsystems assume saturated flow within the region between the streambed and aquifer, or hyporheic nzone. However, pumping next to a stream may cause the region between the bottom of the nstreambed and the aquifer to become unsaturated. When this perching occurs, the flux from the nstream to the underlying aquifer approaches a constant limit. Unsaturated, hyporheic zone flow ntransforms streams from constant head boundaries to specific flux boundaries. This research npresents an analytical solution for drawdown and stream depletion that accounts for unsaturated nhyporheic zone flow. The saturated and unsaturated hyporheic zone flows are linearly superimposed nin the analytical model to account for transient stream disconnection along the length of the stream. nThe effect of unsaturated hyporheic zone flow on the total flux from surface water to groundwater will nbe investigated using the proposed analytical solution.