Surface water-ground water interaction

Background:

Ground-water interaction with surface water is a natural phenomenon dictated by the fact that the two water media are critical components of one system that some workers in this field have described as a three-dimensional watershed. Ground water contribution to surface water can be in the form of base flow to rivers and streams, submerged springs or seepage of ground water into coastal zones. The USGS has estimated the average of ground water contribution to surface water in the US to be over 40% (Ground Water and Surface Water- A Single Resource, USGS Circular 1139, 1998).

In another study; the USGS also estimated the percentage of ground water contribution to selected streams across the country. The estimates reflected a wide range of percentages of contribution as illustrated in Figure 1 (Source: USGS Circular 1139, 1998) below. In carbonate aquifers, such as the Floridan Aquifer System in north Florida, conduit flow during the dry season can produce up to 100% base flow in rivers such as the Suwannee. The implications of this interaction on watershed health and on environmental regulation and protection can not be over-emphasized. Unfortunately, the data needed to quantify such interaction, which is necessary for implementing environmental protection programs, has historically been both difficult and costly to obtain using traditional hydrogeological procedures. This situation may have forced workers in the field but to frequently ignore the fact of interaction and deal with ground and surface water as two separate entities; to the detriment of watershed and ecosystem health and integrity.

To help rectify this situation, the Hydrogeology Section at FGS Hydrogeology Section has directed a significant amount of its research effort to the development and field-testing of innovative approaches to understanding the dynamics of SW/GW interaction. That in turn, will help in providing better estimates of the sources and quantities of water involved and in determining its quality. This information is needed to model the flow of water and to predict the behavior and fate of contaminants in karstic settings. In these settings characterized by conduit flow and multi-porosity, the traditional flow equations (Darcy’s Law) are not applicable, necessitating the development of new models. It is our hope that once these new models are proven effective in the field, they might be used by the regulatory programs and the consulting community to account for ground-water contribution while implementing various environmental programs. Possible examples of such programs include Wetland and Source Water Management and Protection, Waste-Load Allocation including springshed nutrient loading, delineation of springshed boundaries and relative recharge areas, public drinking-water supply well fields and calculation of Total Maximum Daily Loads (TMDL).

The list of projects below is meant to provide the reader with an idea about the overall direction of this research at FGS. Among other approaches, the research seeks to demonstrate the effectiveness of remote-sensing techniques in locating areas of ground water discharge to surface waters. Once such promising areas are identified on a large scale; they can be further “ground-truthed” on a more focused scale, for the purpose of determining the quality and volumes of water involved.

Recent and Ongoing SW/GW Research:

• Conduct thermography and resistivity surveys to increase knowledge of surface-water ground-water interactions; surveyed areas of interest to TMDL studies such as the Ortega River, Escambia Bay, and Lake Barco
• Apply the Wakulla Spring system and the Woodville Karst Plain watershed as a hydrogeological observatory in support of modeling of ground-water flow and contaminant transport in karst terrains, under this task several projects were initiated including the following:



• Installed hydrologic instrumentation through boreholes drilled into spring caves and conduits to measure parameters (i.e., flow, direction, temperature, pressure, specific conductance, etc.) for statistical analysis.
• Deployed microbiological tools into the system’s caves and conduits to grow native bacteria as “bio-films” in an effort to assess the biological communities and understand the relationship between native organisms in the spring and pathogenic organisms introduced from land surface.
• Correlate data with surface processes such as historical rain fall, atmospheric pressure changes, storm events, tidal cycles, etc.
• Establish a relationship between such events and changes in the quality (such as dark water, turbidity or elevated nitrate concentrations) and flow of water issuing from the spring.
• Assess response of spring and aquifer system behavior to last year’s series of hurricanes that impacted the region (planned for publication in a refereed journal).
• Conduct dye tracing to better understand the water budget of the entire Woodville Karst Plain watershed and to specifically assess surface water-ground water interactions.
• Allow prediction of potential impacts on the spring system due to natural and anthropogenic events
(see land use poster on right).
• Calibrate and validate models for ground-water flow and contaminant transport in karst, progress on this project will be reported in the paper being prepared for publication.
• Assist in the development of more accurate approaches to modeling minimum flows in surface waters and minimum levels of water in aquifers to better understand and predict future water needs and its ramifications on the health of ecosystems.
• Application of radium and radon isotope measurements to estimate the rates of ground-water discharge to surface waters via submarine springs or diffuse flow along the coastal zone.
• Assess feasibility of using Landsat imagery to locate offshore sources of potable water.
• Application of thermography, resistivity, side-scan surveys and site investigations to characterize offshore springs.
• Evaluate the effectiveness and resolution of several geophysical techniques (micro-gravity, resistivity, cave radios etc in determining the location and morphology of caves and conduits.
• Evaluate the use of foraminifera as indicators of historical salinity changes and pollution in coastal waters.




For information regarding these projects, contact Dr. Rodney DeHan, FDEP/FGS Hydrogeology Section Outsourcing Manager.