Georgia Water Resources Conference

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Georgia Water Resources Conference

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Georgia Water Resources Institute

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Now showing 1 - 10 of 13

Comparison of Digital Filter Hydrograph Separation with Geochemical Separation

(Georgia Institute of Technology, 2013-04) Mason-Deese, William ; Dowd, John F. ; Cary, Richard H.

Aquatic Stormflow is defined as flow resulting directly from a storm event, while baseflow is thought to be groundwater flow that continuously occurs, most predominantly during non-storm periods. While conceptually these concepts are convenient, it is difficult to ascertain the actual flow paths of each component. In this paper we will compare common digital filters used to estimate baseflow with a geochemically derived baseflow separation. A Dynamic End Member Mixing Analysis (DEMMA) on Panola Mountain, Georgia was used by Cary (2011) to separate four stream flow components using naturally occurring chemical tracers for 22 storm events. DEMMA relies on intensive runoff and chemical sampling, and uses the flow and chemistry hysteresis to separate the hydrograph. Several digital filters were compared to the DEMMA hydrographs. While parameterized differently, each was a recursive procedure that acts as a low pass filter. In general the digital filters over estimate true baseflow for Panola (that is, true groundwater flow), and more closely resemble contributions from subsurface flow (that is soil) pathways. The one parameter filters are insensitive to calibration, although simple to use because the parameter is usually not modified. The two parameter filter (Eckhardt, 2005) was more robust in its range, but sensitive to calibration. This research provides some insight into the flow paths the digital filters may be approximating.
Site Investigation of Southern Historic Cattle Dip Vats

(Georgia Institute of Technology, 2011-04) McKinnon, Robert J. ; Fischer, Teresa A. ; Lodato, Michael N. ; Dowd, John F.

From the early 1900s to 1961, cattle ranchers in Southern states, particularly Arkansas, Georgia, Florida, and Texas, were required to control ticks on cattle to prevent Texas Cattle Fever, a disease that impacted cattle in the South. A United States Department of Agriculture program provided for the construction and operation of over 3,200 cattle dip vats. Solutions containing arsenic were initially used, until the ticks developed a resistance to the arsenic solution. Around the mid-1940s, arsenical pesticides were replaced by organochlorinated pesticides. The historic operations of cattle dip vats have lingering impacts on groundwater and soil that pose a threat to human health and the environment. Geosyntec Consultants has contracted with the Florida Department of Environmental Protection to evaluate groundwater and soil impacts associated with historic cattle dip vats that are located in the Florida State Park System. A multi-faceted approach was used to characterize contaminant migration in groundwater and soils with respect to historical site operations, archaeological preservation, and environmental health. Several case studies at state parks located across Florida are presented, including methods of delineation and remediation. The approaches employed in these studies will assist others in future investigations of historic cattle dip vats that exist across the South, including Georgia.
Determining Watershed Flow Pathways Using Geochemistry and Timing

(Georgia Institute of Technology, 2011-04) Cary, R. H. ; Dowd, John F. ; Peters, N. E.

Investigating storm runoff generation in watersheds is an area of ongoing hydrologic research. Geochemical tracer studies, such as static end-member mixing analysis (EMMA) and hysteresis loop analysis, have been used to evaluate these processes. While EMMA can assess the relative input of flow pathways for individual stream water quality samples collected during a storm, it cannot quantify their contributions continuously. Hysteresis loops of stream discharge versus geochemical tracer concentration can be used to estimate relative inputs of basic end-member pathways, but this approach only suggests the timing and dominance of flow pathways and these patterns alone cannot quantify their contributions. We propose a new method that incorporates both hysteresis loops and geochemical tracer studies to quantify runoff contributions from watershed flow pathways during a storm. The approach involves estimating relative tracer concentrations of four end-members, along with estimating the percentage of total stream discharge from each end-member. The method has been applied to a 22 year dataset from Panola Mountain Research Watershed, Georgia and has identified two distinct watershed responses to rain events. The responses appear be related to a threshold of 50-60mm of total rain.
Beyond Correlation: the Search for Causal Relationships Between Flow Percentiles and Watershed Variables

(Georgia Institute of Technology, 2011-04) Ssegane, Herbert ; Tollner, E. W. ; Mohamoud, Yusuf ; Rasmussen, Todd C. ; Dowd, John F.

The study explored use of causal feature selection algorithms to select dominant watershed variables that drive high, medium, and low flows. A two step approach was implemented. The first step minimized variable redundancy by examining variable relevance, variable redundancy, and conditional relevance of variable pairs whose correlation was greater than 0.9. The second step used six algorithms that seek to reconstruct a Bayesian network structure around a target variable for each flow percentile. Nineteen (19) flow percentiles were used to characterize high, medium, and low flow conditions of 26 Piedmont watersheds in the Mid-Atlantic. The algorithms included: (1) Grow-Shrink (GS); (2) interleaved-Incremental Association Markov Boundary (interIAMB) (3) Incremental Association Markov Boundary with Peter-Clark (IAMBnPC); (4) Local Causal Discovery (LCD2); (5) HITON-PC; and (6) HITON-MB. A new method was developed to quantify the reliability of each algorithm and its performance was compared to existing reliability methods. The effect of the initial number of variables on the final variable set selected by each algorithm was tested. Fusion of the algorithms was used to determine the overall dominant features for each flow percentile.
A Mechanism for Storm Runoff Generation During Large Rainfall Events

(Georgia Institute of Technology, 2007-03) McKinnon, R. J. ; Dowd, John F. ; Endale, Dinku M.

Flowpaths of stormwater from upland areas have long been the subject of major debate. A series of subsurface gutter experiments, situated on the mid- slope of a Piedmont catchment, were conducted to investigate a potential mechanism for the rapid mobilization of storm runoff from the unsaturated zone. Gutters were 1.45 m long and installed approximately 10 cm below the ground surface. Direct surface runoff was excluded from entering the gutters. Nearly a year of natural rainfall monitoring data showed a close relationship between rainfall intensity and the resulting runoff in the subsurface gutters. The gutter response closely followed the onset of intense rainfall and likewise “switched off” with the cessation of storm events. This behavior is not indicative of a saturated subsurface flow mechanism. Stable isotope analysis of runoff samples demonstrated that stormflow was comprised primarily of “old water,” which is water that was in the soil before the initiation of rainfall. Thus, the traditional explanations, macropore flow and overland flow, could not have been the dominant processes because they produce mainly “new water”. The data suggest that runoff from large storm events occurs when high intensity rainfall generates pressure waves that rapidly travel through the soil and induce pre-event water. Some hydrologists refer to this as a kinematic process. Research on this process at the field level will lead to understanding of stormflow pathways and the associated potential for transport of pollutants at the landscape scale.
Observations in Granite Quarries Facilitate Understanding the Hydrogeology of the Georgia Piedmont

(Georgia Institute of Technology, 2007-03) Wenner, David B. ; Dowd, John F.

Because the crystalline bedrock of the Piedmont Province in Georgia is so poorly exposed, it can be difficult to readily understand how water flows in the subsurface and why bedrock well yields in the Piedmont are so variable. Quarries provide an easy way to visually observe how the hydrogeology in this terrain operates. There are numerous quarries in the Elberton area and elsewhere that show excellent exposures of both the overlying regolith and bedrock, the principal components of the aquifer system of the Piedmont. Within the exposed bedrock, it is readily apparent that water is transmitted in only a few fractures. Most fractures show evidence of little or no water movement. Fractures that yield relatively large amounts of water are mostly shallow, are generally extensive, and appear to be connected to the overlying regolith, the principle reservoir for groundwater. Deeper fractures appear to transmit little water, perhaps because they are less abundant and have smaller apertures due to the overlying lithostatic load. Even those fractures that appear to transmit relatively large amounts of water may actually have very small apertures. Calculations using the cubic flow law indicate that fracture apertures for typical flow rates are quite small, with very small fracture porosities, despite their appearance at the surface of the quarry to the contrary.
Evaluation of the Micropurge (Low-flow) Method to the Packer Method of Groundwater Sampling of Fractures in Crystalline Bedrock of the North Georgia Piedmont (Poster)

(Georgia Institute of Technology, 2001-03) Feild, James B. ; Dowd, John F.

In recent years, the micropurge (or low-flow) method of collecting groundwater samples has become popular. The advantages of the method include low cost of purge water disposal due to small volumes, minimal colloidal material in the samples, and shorter sampling time The method assumes that water will flow horizontally from the formation to the pump in the well with minimal mixing of casing water. Normally it is limited to use in wells with short screens, and pump rates low enough to minimize drawdown and volatilization of contaminants. Puls et al. (1996) recommends the use of packers for wells with long screens or open borehole fractured bedrock wells. Without the use of packers, the risk of spreading contamination to clean portions of the aquifer is high. In a fractured-bedrock aquifer at a contaminated site near Danielsville, Georgia, groundwater samples were collected from fractured-bedrock wells using the micropurge method and the packer method. Comparison of the two methods indicate that formation water was not sampled when the micropurge method was used.
Krypton Tracer Test to Characterize the Recharge of a Highly Fractured Aquifer in Lawrenceville, Georgia

(Georgia Institute of Technology, 1999-03) Uddin, Momin K. ; Dowd, John F. ; Wenner, David B.

The City of Lawrenceville uses a high yield well located in Rhodes Jordan Park to supplement its drinking water supplies. This area has been investigated by the U.S. Geological Survey, the University of Georgia, and the City of Lawrenceville since 1994 to identify the fracture system and recharge to the well. This study reports on gas tracer tests that provide additional insight into the natural flow field at the site. The tracer experiment involved diffusing krypton gas into a bedrock observation well on-site and monitoring the response of krypton in the production well. This procedure was performed using bedrock observation wells situated in different directions from the production well. The results indicate that a larger fracture aperture exists in the direction that is at right angles to the regional strike.
Evaluation of a BTEX Plume in Fractured Crystalline Rock in the Georgia Piedmont

(Georgia Institute of Technology, 1999-03) Feild, James B. ; Dowd, John F.

Water flow and chemical transport in fractured media has become a major concern in recent years. Traditionally, these problems have been viewed and modeled as flow through pipes. However, this approach is limited because it is very difficult to predict the orientation, size, and length of the fractures, hence the direction and velocity of the petroleum hydrocarbons. Groundwater sampling of saprolite, residential, and bedrock wells at a site near Danielsville, Georgia has revealed the presence of petroleum constituents in the aquifer system beneath the site. The site has a documented history of spills over the past thirty years. Groundwater sampling of some of the residential and monitoring wells in the vicinity of the site has indicated benzene contamination ranging from non-detectable to approximately 2500 parts per billion. It has been established from groundwater samples collected from bedrock that contamination has migrated from the saprolite into the fractured bedrock aquifer. A suspected pathway of contaminant migration could possibly be an improperly cased bedrock well. Physical and chemical conditions at the site have been modeled using a three dimensional visualization program. In this paper, we show how the use of this tool leads to understanding of the fracture flow system, and how this knowledge can be used to prepare a more comprehensive, physically based fracture flow model.
Use of Ground-penetrating Radar to Characterize Hydrostratigraphic Units Within a Georgia Coastal Plain Province

(Georgia Institute of Technology, 1997-03) Feild, James B. ; Johnson, West W. ; Sermon, Nina ; Burkingstock, Bryan K. ; Dowd, John F. ; Garrison, Ervan G. ; Bush, Parshall B.

The fate of pesticides/herbicides in a watershed depends upon important hydrologic pathways. In the Coastal Plain of Georgia, the presence and extent of semi-confining lithologic layers dictates the potential for contaminant movement into the local water table. This study used ground penetrating radar to characterize subsurface hydrostratigraphic units on a Coastal Plain study site. Groundwater monitoring wells were installed at various locations at the site, and soil samples were collected and described. The site lithology is characterized by a fine grained quartz sand which is underlain by a clay-rich indurated sandy loam (CRISL). These soil samples are only point descriptions and do not describe the continuity of the layer. Therefore, ground penetrating radar was utilized to map the lithological features of the CRISL and determine the continuity of this layer. Analysis of the ground penetrating radar data suggests the CRISL (semi-confining layer) is discontinuous, and the discontinuity is elongated at the region along the ephemeral stream that drains the watershed. This geometry may be used to accurately illustrate unsaturated/saturated flow in a contaminant transport model.