Measuring Floodplain Hydraulics of Seco Creek and Medina River Where They Overlie the Edwards Aquifer – Final Report

Author Green RT, Bertetti FP, McGinnis R and Prikryl J (Southwest Research Institute®)
Year 2012
Description Study of the hydraulic relationships between Seco Creek, the Medina River, and the Edwards Aquifer
Report Number SwRI Project 20-16488
Publisher Southwest Research Institute®
Location Edwards Aquifer, Balcones Fault Zone, San Antonio Segment, Medina County
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Medina County, Texas, hosts significant groundwater resources in a number of alluvial and consolidated rock aquifers. The most significant of these is the Edwards Aquifer, which spans the central portion of the county from its western to eastern boundaries. An estimated 15-25 percent of the recharge of the San Antonio segment of the Edwards Aquifer has been attributed to recharge that occurs in Medina County (Hamilton et al., 2008). Sources of recharge to the Edwards Aquifer typically consist of (i) precipitation on the recharge zone (i.e., autogenic recharge), (ii) surface water focused in river and stream beds (i.e., allogenic recharge) and (iii) as subsurface interformational flow from upstream aquifers. Discharge occurs by spring flow, pumping, and interformational flow to downstream aquifers. To effectively manage the Edwards Aquifer, the water budget must be adequately quantified, and to calculate the water budget within acceptable limits, recharge and discharge of the aquifer must be adequately characterized…

Effective management of the Edwards Aquifer requires that its water budget be accurately known. Central to calculation of the water budget of the Edwards Aquifer are recharge and discharge distributions and rates. Current estimates of recharge of the Edwards Aquifer by the Seco and Hondo Creeks and Medina River (i.e., annual medium of 174,600 acre-ft) are predicated on the assumption that underflow in the Seco and Hondo Creeks and Medina River floodplains is negligible and that recharge from these rivers is accurately measured using river flow gauges.

This investigation was undertaken to improve understanding of the hydraulic importance of the Seco Creek and Medina River to the Edwards Aquifer and to evaluate whether this conceptualization of recharge by these rivers is valid. As a secondary objective, hydrogeological and water chemistry data were examined to ascertain the nature of groundwater flow paths in the Edwards Aquifer in Medina County.

In general, the hydrochemical analyses conducted in this study provide support for east to west flowpaths in northern Medina County along fault lines previous investigators  proposed (Maclay, 1995; Groschen, 1996; Clark and Journey, 2006). These flowpaths are evaluated both with faults as barriers to flow and with faults not acting as barriers; however, it is likely that faults do play a role in the actual paths for flow within the Edwards Aquifer in Medina County. With the barrier effect added, the chloride and magnesium maps highlight compartmentalization of the aquifer along fault lines, and the maps remain quite consistent with patterns predicted by proposed east to west flowpaths in the north that tum south and east near Sabinal. Adding the barriers does not eliminate the observed plume of more dilute water in the confined zone between Hondo and Castroville.

Hydrochemical data analyses also indicate at least two flowpaths in northern Medina County that flow westward into the confined zone before turning south and eastward near the Medina-Uvalde County boundary and the city of Sabinal, where they mix with eastward flow from Uvalde County. These flowpaths are similar to those Maclay ( 1995), Groschen ( 1996), and Clark and Journey (2006) proposed. The data appear to indicate a shorter flowpath along the southern part of the recharge zone that turns south and eastward between Hondo and Castroville near the Elm-Quihi-Hondo Creek confluence.

This flowpath is consistent with the model flowpath of Maclay ( 1995) and data in Groschen ( 1996), but may not be consistent with the south-central flow path Clark and Journey (2006) proposed. The data indicate an isolated block of higher TDS water just north of Hondo. This isolated block is consistent with data from Groschen ( 1996) and Fahlquist and Ardis (2004). Hydrochemical data indicate a zone of Trinity and Edwards Aquifer mixing west and southwest of Medina Lake near the Medina-Bexar County boundary. These data appear to be consistent with data from Clark and Journey (2006), but are not consistent with their proposed flow path for that area.

Data for the Carrizo-Wilcox Aquifer suggest some recharge connection to an Edwards Aquifer-like source, but more study is required to draw any conclusion regarding the interaction between the Edwards and Carrizo-Wilcox Aquifers south of Medina County.

When combined with the hydrostratigraphic framework of the area, water chemistry, hydraulics, and subsurface imaging can provide the basis to develop a conceptual model of the hydraulic boundary at the southern edge of the Edwards Aquifer recharge zone in central Medina County.

Geologic structure, subsurface imaging, groundwater and surface-water elevations, and water quality of the Leona Formation sediments in central Medina County and the Medina River were examined to evaluate the hydraulic relationships between the Leona Formation sediments and Medina River floodplain systems and the Edwards Aquifer. Seco Creek and Medina River were selected for focused study because of their importance to recharge of the Edwards Aquifer and their suspected importance as modes of discharge from the Edwards Aquifer. In particular, geophysical imaging of the subsurface of the floodplains of Seco Creek and Medina River was conducted to ascertain evidence of paleo-channel deposits or preferential flow pathways developed in the floodplain sediments.

A series of creeks, including Seco, Parker, Live Oak, Hondo, Verde, Elm, and Quihi, provide surface drainage for the Leona Formation in central Medina County. Based on the results of this investigation, paleo-stream channels in the Seco-Parker Creek floodplain are interpreted to flow south until reaching Highway 90, at which point they flow east eventually coalescing with paleo-stream channels associated with the Live Oak floodplain. This interpretation was based on the detection of a significant paleo-stream channel in the Seco-Parker Creek floodplain to the north, but no paleo-stream channel to the south. Consistent with this conceptualization, paleo-stream channels associated with Hondo, Verde, Elm, and Quihi Creeks were interpreted to flow south and eventually coalesce with the Live Oak paleo-stream channel into a single channel about 8-9 miles south of Hondo. This later interpretation was corroborated with hydrochemical analysis, but not verified or quantified by subsurface geophysical imaging.

Significant paleo-stream channel deposits were detected in the Medina River floodplain. The width of the channel deposits exceeds 1 mile. Local well logs confirm the depth of the channel deposits is uniform at about 7 feet. In the absence of an aquifer test, hydraulic properties for the channel deposits were estimated using documented values for a coarse gravel. Using this information, the capacity for underflow in the Medina River floodplain is estimated at 17,500-175,000 acre-ft/year compared with the average annual surface flow of 146,000 acre-ft/year for the Medina River.

In summary, this investigation determined that there is significant underflow in the Medina River floodplain, negligible underflow in the southern Seco Creek floodplain, and potential underflow in the Live Oak/Hondo Creek floodplain, although the magnitude of this potential remains unquantified at this time.