Date of Award
Master of Science (MS)
Murdoch, Lawrence C
Falta , Ronald W
Moysey , Stephen
Groundwater discharge is a key component in understanding the interaction between groundwater and surface water. It provides baseflow to streams and can transport nutrients and contaminants to related aquatic ecosystems. It can also influence fish behaviour, spawning, migration and provide areas of refuge during summer and winter months. There are a variety of techniques that are used to characterize the magnitude of groundwater discharge including seepage meters, field permeameters, and piezometers and manometers, however the controls for the magnitude and distribution of discharge remain poorly understood. The purpose of this study was to conduct field investigations complimented with numerical analysis to gain a better understanding of the possible factors that may control the magnitude and distribution of groundwater discharge.
Field investigations were conducted at two rivers, the Bogue Phalia, MS (Summer 2007) and Eighteen Mile Creek, SC (Summer 2008) for the purpose of characterizing the magnitude and distribution of groundwater discharge. The magnitude and distribution of groundwater discharge was determined for both rivers using modified pan-and-bag seepage meters. Discharge measurements were complimented with measurements of stream-bed hydraulic head gradient, stream-bed hydraulic conductivity, grain-size analysis, stream flow, and sediment cores.
Field results at both streams indicated that there is a dominate pattern of groundwater discharge with greatest discharge occurring in the center of the stream where bed sediments were coarsest and decreasing towards the banks where sediments were finer. Localized variations to this dominant pattern were identified at both streams but were isolated to small reaches of stream. The dominate pattern of discharge has also been identified at other rivers and streams using similar techniques including Leary Weber Ditch, IN, Maple Creek, NE, Twelve Mile Creek, SC, and West Bear Creek, NC. This pattern is interesting because it differs markedly from the dominant pattern of groundwater discharge that has been identified at lakes, where the groundwater discharge is greatest along the banks and decreases away from shore. This pattern has been identified at lakes using both field techniques and numerical analysis representing lakes as a constant head boundary.
To better constrain the possible factors that may control the distribution of groundwater discharge, numerical analysis was conducted using the finite-difference groundwater model, MODFLOW in conjunction with ModelMuse. Model parameters were scaled to a characteristic flux and length to create dimensionless variables and generalize results. Numerical analysis of idealized stream scenarios indicate that the most likely control for the distribution of groundwater discharge is heterogeneities in stream bed sediments. A one order of magnitude difference in the hydraulic conductivity of sediments occurring along the bank compared to those occurring at the center of the stream produced distributions of groundwater discharge that were nearly identical to the dominant pattern identified in the field. Another factor that was identified as a possible control was evapotranspiration of plants occurring along the banks of streams. Numerical analysis indicated that evapotranspiration rates based on the consumptive use of groundwater by phreatophytes can reduce fluxes along the banks of streams but also creates a reduction in average groundwater discharge to the stream, which is unjustified based on a simple mass balance of an idealized aquifer. To replicate the dominant pattern of groundwater discharge observed in the field, evapotranspiration would have to be approximately 80 times the recharge rate for the region. Greater values of evapotranspiration still create ever larger losses in the average discharge to the stream and are considered unjustified.
Results from this study can be applied to multiple aspects of research that occur in streams. Identifying areas of high discharge can help conservationist identify and protect possible fish spawning areas, refuge habitats in summer and winter seasons, as well as monitoring behaviour and habitat selection. It may also help identify areas where contaminants or nutrients are discharging in higher concentrations to streams reducing the need for multiple sampling locations in streams, which could save time and money for consultants in both the private and public sectors.
Gebhard, Curtis, "Identification of Patterns and Controls of the Distribution of Groundwater Discharge to Streams" (2010). All Theses. 1029.