Hydrologic Effects of Well-Field
Operations in a Wetland,
Dade County, Florida
By Roy S. Sonenshein, U.S. Geological Survey; and
Ronald H. Hofstetter, University of Miami
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 90-4143
Prepared in cooperation with the
SOUTH FLORIDA WATER MANAGEMENT DISTRICT
and the
METRO-DADE WATER AND SEWER AUTHORITY
Tallahassee, Florida
1990
U.S. DEPARTMENT OF THE INTERIOR
MANUEL LUJAN, JR., Secretary
U.S. GEOLOGICAL SURVEY
Dallas L. Peck, Director
ABSTRACT
Water-level, canal stage and discharge, and rainfall data collected in a wetland in Dade County, Florida, were analyzed to determine the effects of pumping at the Northwest Well Field on water levels in the wetland. The Northwest Well Field is the first major well field in south Florida to be operated in a wetland, away from saltwater intrusion and the potential for contamination caused by urbanization.
Duration curves were used to analyze trends in water levels for seven observation wells near the Northwest Well Field. One observation well is 5.5 miles north of the well field, three wells are outside the cone of depression of the well field, and three are within the cone of depression. The water-level data were analyzed for four time periods that were determined by a double-mass analysis of cumulative rainfall and cumulative canal discharge. Before 1984, water levels in all seven wells were above land surface 25 to 50 percent of the time. Since the well field began operating in 1984, water levels in the three wells within the cone of depression have been above land surface less than 1 percent of the time. Water levels at the four wells outside the cone of depression showed no effect from pumping at the well field.
Water levels have declined in 30 percent of the 65-square mile study area since the well field began operating. In 15 percent of the area, water levels have been lowered below land surface.
CONTENTS
Abstract 1
Introduction 1
Purpose and scope 3
Description of study area 3
Previous investigations 3
Acknowledgments 3
Data compilation and analysis 3
Rainfall 3
Rainfall-discharge relation 4
Well-field pumpage 4
Water-level trends 7
Background conditions 7
Wells outside the cone of depression 7
Wells within the cone of depression 8
Effects of pumping on water levels in the wetland 9
Summary 15
References cited 15
FIGURES
1. Map of study area showing data-collection sites, section A-A',
and region of wetlands dewatered by pumping 2
2-3. Graphs showing:
2. Monthly rainfall at Miami International Airport and mean daily
pumpage by month from the Northwest Well Field, May 1983 to
September 1987 5
3. Average monthly rainfall at Miami International Airport and average
mean daily pumpage by month from the Northwest Well Field, January
1984 to September 1987 6
4. Double-mass curve showing relation between cumulative rainfall at Miami
International Airport and cumulative discharge in Tamiami Canal at Coral Gables,
1961 to 1987 7
5-8. Hydrographs of daily maximum water levels in observation wells:
5. G-968 and G-972, October 1960 to September 1987 8
6. G-975 and G-1488, October 1960 to September 1987 9
7. G-974 and G-976, October 1960 to September 1987 10
8. G-3253, December 1981 to September 1987 11
9-12. Water-level duration curves for:
9. Wells G-968 and G-972 12
10. Wells G-975 and G-1488 13
11. Wells G-974 and G-976 14
12. Well G-3253 15
13. Vertical profile along section A-A' showing water levels for
April 1982, October 1982, October 1987, and April 1988 16
TABLE
1. Available ground-water level, surface-water stage and discharge, and rainfall data
at data-collection sites in the study area 4
SUMMARY
Water-level duration curves and double-mass curves were used to determine the effects of water-management practices and of pumping from the Northwest Well Field on water levels in the vicinity of the well field. The doublemass curve relating cumulative rainfall and cumulative discharge of the Tamiami Canal at Coral Gables indicated changes in water-management practices beginning in 1971 and again in 1978. This curve also showed the decrease in discharge of the canal in May 1983 when the well field began pumping. The three breakpoints in the double-mass curve (1970, 1977, 1983) were used to determine the four time periods used for the duration-curve analysis. Analysis of the duration curves indicated that water levels at three of the six monitoring wells in the study area were affected by well-field pumping. In the center of the well field, water levels have been lowered by more than 6 feet in both wet and dry seasons.
Water levels have been affected by well-field pumping in an area encompassing almost 20 mi2 (30 percent of study area) around the well field. This region is divided into two almost equal parts. The first part is the region within and immediately around the well field where the wetland has been dewatered and the land is now dry. Surrounding this region is an area where water levels are not below land surface all of the time but have been lowered by pumping. The area dewatered by pumping at the well field is about 10 mi2 (15 percent of the study area) and is no longer considered a wetland.
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