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Scientific Investigations Report 2011–5054

Prepared in cooperation with the Maryland Department of the Environment,
Maryland Department of Natural Resources,
Maryland Department of Agriculture,
Dorchester Soil Conservation District, and the
U.S. Fish and Wildlife Service

Geology, Hydrology, and Water Quality of the Little Blackwater River Watershed, Dorchester County, Maryland, 2006–09

Brandon J. Fleming, Benjamin D. DeJong, and Daniel J. Phelan

ABSTRACT

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The Little Blackwater River watershed is a low-lying tidal watershed in Dorchester County, Maryland. The potential exists for increased residential development in a mostly agricultural watershed that drains into the Blackwater National Wildlife Refuge. Groundwater and surface-water levels were collected along with water-quality samples to document hydrologic and geochemical conditions within the watershed prior to potential land-use changes.

Lithologic logs were collected in the Little Blackwater River watershed and interpreted with existing geophysical logs to conceptualize the shallow groundwater-flow system. A shallow water table exists in much of the watershed as shown by sediment cores and surface geophysical surveys. Water-table wells have seasonal variations of 6 feet, with the lowest water levels occurring in September and October. Seasonally low water-table levels are lower than the stage of the Little Blackwater River, creating the potential for surface-water infiltration into the water table.

Two stream gages, each equipped with stage, velocity, specific conductance, and temperature sensors, were installed at the approximate mid-point of the watershed and near the mouth of the Little Blackwater River. The gages recorded data continuously and also were equipped with telemetry. Discharge calculated at the mouth of the Little Blackwater River showed a seasonal pattern, with net positive discharge in the winter and spring months and net negative discharge (flow into the watershed from Blackwater National Wildlife Refuge and Fishing Bay) in the summer and fall months. Continuous water-quality records showed an increase in specific conductance during the summer and fall months.

Discrete water-quality samples were collected during 2007–08 from 13 of 15 monitoring wells and during 2006–09 from 9 surface-water sites to characterize pre-development conditions and the seasonal variability of inorganic constituents and nutrients. The highest mean values of nitrogen are found in the deep groundwater system, with relatively low values in the water table. Surface-water-quality samples in the lower half of the basin show a significant increase in inoganic seawater constituents, especially in summer, corresponding with net negative discharge from the Little Blackwater River.

Samples also were collected from nine wells and four surface-water sites for pesticides in June 2008. The herbicides atrazine, metolachlor, and simazine, and the insecticide fipronil were detected at each of the four surface-water sites, with concentrations less than 2 micrograms per liter. Concentrations of pesticides found in groundwater were typically one to two orders of magnitude lower than pesticide concentrations found in surface water of the Little Blackwater River.

Seasonal hydraulic-gradient reversals between the shallow groundwater system and the Little Blackwater River, coincident with the inflow of brackish water from Fishing Bay and Blackwater National Wildlife Refuge, indicate a potential for saltwater intrusion into the water table. The likelihood of saltwater intrusion into the water table is further supported by high chloride concentrations observed in water-table wells near the Little Blackwater River.

Posted May 2011

For additional information contact:
Director
U.S. Geological Survey
MD-DE-DC Water Science Center
5522 Research Park Drive
Baltimore, MD 21228
(443) 498-5500

http://md.water.usgs.gov

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Suggested citation:

Fleming, B.J., DeJong, B.D., and Phelan, D.J., 2011, Geology, hydrology, and water quality of the Little Blackwater River watershed, Dorchester County, Maryland, 2006–09: U.S. Geological Survey Scientific Investigations Report 2011–5054, 82 p., available at http://pubs.usgs.gov/sir/2011/5054/.



Contents

Abstract

Introduction

Purpose and Scope

Description of Study Area

Methods

Groundwater and Surface-Water Monitoring Methods

Groundwater and Surface-Water Sampling Methods

Geology

Miocene Stratigraphy

Pleistocene-Holocene Stratigraphy

Hydrology

Groundwater-Flow System

Surface-Water System

Water Quality

Data Quality Assurance

Blanks

Duplicates

Ion Balances

Groundwater Quality

Inorganic Constituents

Organic Constituents

Surface-Water Quality

Inorganic Constituents

Organic Constituents

Summary and Conclusions

Acknowledgments

Selected References


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