Guilford County Groundwater Monitoring Network Status Report

Transcription

1 Guilford County Groundwater Monitoring Network Status Report HERA Team Division of Environmental Health Department of Public Health June 2007

2 Guilford County Groundwater Network Status Report INTRODUCTION The industrial development and population growth in Guilford County have resulted in an ever-increasing demand for water. In the rural areas of the county, groundwater remains the water supply resource for domestic use. In 1990, the population of Guilford County was 347,420 and approximately 74,460 residents used groundwater from wells tapping the fractured crystalline rock aquifer system underlying the county for their domestic water supplies (US Bureau of the Census, 1992). It is estimated that the population of the county in 2006 was approximately 452,000. Approximately 135,000 residents or 30 percent of the population in the county use groundwater for domestic use. The number of residents depending upon groundwater for potable supplies has doubled in the last 15 years and will continue to increase with population growth in the county. In some areas of the county, the wastewater from the residences using groundwater is discharged to the city sewer system and is not recharged back to the source area. This has resulted in a net loss of groundwater resources. With the steady population growth in the county, the demand for groundwater has increased. In the last few years, many community wells with daily usage of more than 10,000 gallons have been installed in many new developments in the county, particularly in the northwestern part of the county including Summerfield, Oak Ridge, and Stokesdale areas. Because the amount of groundwater in the bedrock aquifers available in the county for potable water is largely unknown, the availability of groundwater as a present and future resource has been a concern for the water supplies in the suburban communities. Therefore, it is necessary to establish a long-term monitoring network across the county to monitor and determine whether the increasing new developments in the county will impact the availability of groundwater resources. The results obtained from the long-term groundwater monitoring would benefit county government for the policy making for new development and permitting process in the county and area developers for considering groundwater resources for the new developments. PREVIOUS STUDIES There have been no previous studies to monitor long-term groundwater level and impact of development in the rural areas on the availability of water supplies in Guilford County. Mundorff (1948) analyzed the data from 377 wells in Guilford County to identify the relations between well yields, rock units, and topographic settings of the well sites. Daniel and Sharpess (1983) included some wells in Guilford County in a study of groundwater resources in the upper Cape Fear River Basin. Daniel and Payne (1990) mapped the hydrogeologic units in Guilford County as part of hydrogeologic unit mapping in the Piedmont and Blue Ridge Province of North Carolina. Floyd and Peace (1974) investigated groundwater resources as part of their studies of groundwater resources in the upper Cape Fear River Basin. 2

3 Daniel and Harned (1988) studied groundwater recharge to and storage in the regolithfractured crystalline rock aquifer system in Guilford County. They concluded that the recharge rates are highest for basins in the northern and northwestern part of the county and lowest in the southern and southeastern parts of the county. The recharge rate is related to the hydrogeologic unit of a specific area. Spruill and others (1997) investigated radon in groundwater in Guilford County. Their studies indicated that the highest radon activities occurred in younger granites (igneous, felsic intrusive rocks) in the northwestern part of the county and lower activities occurred in the older metamorphic volcanic units in the central and southern portion of the county. GENERAL DESCRIPTION OF GUILFORD COUNTY Guilford County covers approximately 658 square miles in the central part of the Piedmont Province. It is estimated that the population in the county in 2006 was approximately 452,000 according to the Piedmont Triad Council of Governments. The topography of the county consists of low, rounded hills and long, northeast-southwest trending ridges with up to a few hundred feet of local relief. Generally, the elevation of land surface decreases southeasterly in the county. The climate of Guilford County is typed as humid-subtropical. Mean minimum January temperatures range from 31 to 33 F, whereas mean maximum July temperatures range from 87 to 89 F. Average annual precipitation varies across the county from 43 to 48 inches (Kopec and Clay, 1975). The lowest rainfall occurs in the northern and northwestern parts of the county; the highest rainfall occurs in the southern and southeastern parts of the county. Geologically, Guilford County lies within the Charlotte Slate Belt. Metamorphic and igneous crystalline rocks are mantled by varying thickness of regolith (Fig. 1). Felsic igneous intrusive rocks underlie the northwestern part of the county. The southwestern, central, and northeastern parts of the county are underlain primarily by meta-igneous rock of felsic and intermediate composition. Meta-volcanic rocks underlie south and southeast of the county. Under natural conditions, recharge to the groundwater system in the county is mostly from infiltration of precipitation. During dry seasons, recharge to the bedrock aquifer system may be derived from the stream systems. MONITORING NETWORK DESCRIPTION A total of eight (8) stations were installed across the county for this monitoring network (Fig. 2). Each well site was selected to represent the area and to minimize the influence of any existing water supply wells nearby. These stations are located on public properties owned by Guilford County or the City of Greensboro, which allow the Health Department to conduct the monitoring activities easily. Seven local well drilling companies (see acknowledgement) donated their well construction services for the groundwater network project. The water supply well for former Monticello-Brown Summit Elementary School is used as a station for this 3

4 Fig. 1. Geology Map of Guilford County (Spruil and others, 1997) project because this well is not in use for the new school. WELL CONSTRUCTION All wells were drilled and constructed by seven local well drillers. These wells were constructed according to NCAC 2C Well Construction Standards and Guilford County Well Rules. PVC and galvanized steel were used for the well casing at all stations and steel casing extends approximately three (3) feet above ground surface to support an aluminum box for the data logger (Fig. 3). The depths of these wells range from 100 to 180 feet below the ground surface (Table 1). These wells were drilled into bedrock and the casing was set in the bedrock. Bedrock was encountered at depths from 12 feet at Prison Farm station to 80 feet at Fire Station 45. The depths of the water table after the well completion ranged from 10 feet at Prison Farm Station to 39 feet at Jack Dent Park station in Summerfield. The water table in the northwestern part of the county is deeper than that in other areas (Table 1). 4

5 Fig. 2 Locations of monitoring stations NETWORK STATIONS At each station, a metal plate is attached to the top of the well casing to support the data logger. An aluminum box and a lock are used to secure the cable and data logger (Fig. 3). The stations are locked after each monitoring activity. The State Division of Water Quality provided the metal plates and the aluminum boxes. 5

6 Fig. 3 Monitoring Station at Hagan Stone Park (noted this is a composite photo) To run the setup, the depth of the water table was measured with a measurement tape. The transducer (probe) was lowered to the well. The depth of the transducer depends on the depth of the water table and the detection range of the probe, and is set 7.5 feet below the water table for WL model or 15 feet for WL16S-030 and WL models (Table 2). The length of the cable used to connect the transducer and the data logger depends on the depth of the transducer. The depth of the water table from the actual tape measurement was used for calibration to establish the settings so that the depth of the water table from the transducer was the same as the actual tape measurement. The instrument was set to record the depth of the water table below the top of casing hourly. Initially, transducers with 15-foot detection range were installed at all stations. Because the depth of the water table fluctuated at some stations beyond the detection range of the probe, 15-foot transducers were replaced with 30-foot range transducers at Gibson Park and Fire Station 45. With additional monitoring events, the data obtained from Prison Farm and Fire Station 45 indicated that the records of the 15-foot range transducers were inconsistent. According to the manufacturer, WL15 series are not stable and WL16S model is a better transducer. The transducers at Prison Farm and Fire Station were upgraded to the L16S model in November The records obtained from the February 2007 monitoring event indicated the WL16S model has generated good monitoring results. Eventually, all WL15 model transducers will be upgraded to WL16S models. 6

7 Table 1 Guilford County Groundwater Monitoring Network Stations Station Kernersville Summerfield Jamestown Pleasant Garden McLeansville Liberty Ellon (GC) Browns Summit Triad Park Jack Dent Park Gibson Park Hagan Stone Park Fire Station 37 Fire Station 45 Prison Farm Monticello School Well Diameter (in) Well Depth (ft) Depth to Bedrock (ft) Casing Material PVC & PVC & steel Steel PVC & steel PVC & steel PVC & steel PVC & steel Steel steel Casing Depth (ft) Grout Material cement cement cement cement Grout Depth (ft) Depth of Water Table (ft) Water Zone Depth (ft) Constructed by D&Y Well Drilling, Inc. McCall Bothers, Inc. Simmons Well Drillers, Inc. Built-Rite Well Drilling Co. Jones Well & Pump Co. W.W. Maness & Sons, Inc Yadkin Well Co., Inc. Table 2 Settings of Monitoring Stations Station Kernersville Summerfield Jamestown Pleasant Garden McLeansville Liberty Ellon (GC) Browns Summit Triad Park Jack Dent Park Gibson Park Hagan Stone Park Fire Station 37 Fire Station 45 Prison Farm Monticello School Probe Model WL WL WL WL WL16S-030 WL WL16S-030 WL Initial Water Level (ft) Probe s Range (ft) Probe Depth (ft) Installation Date 10/2/02 10/2/02 04/15/03 5/16/03 12/2204 7/1/03 12/22/04 Re-setup Date 1/23/06 2/22/07 1/20/06 11/30/06 1/20/06 11/30/06 1/20/06 7

8 MONITORING ACTIVITIES Each station was visited within a week after the initial setup to check the recording functions. If the reading from the probe was different from the actual tape measurement, the settings were adjusted accordingly to match the true water table depth. Each station has been visited quarterly and after all stations were completed, they have been visited on the same day. During the quarterly visits, the depths of the water table recorded in the data loggers are downloaded to a laptop (Fig 3). A measurement tape is lowered to the wells to check the depth of the water table. If the depth of the water table is inconsistent with the actual measurement at any stations, the settings are adjusted to reflect the true water table depth. After the downloading, the memory of the data logger is cleared for new records and a new 9-volt battery is installed. The field activities at each station are documented for future reference after every monitoring event. DATA PROCESSING The data downloaded from the data loggers are imported to a Microsoft Excel spreadsheet. Each hourly record includes the date/time and the depth of the water table. A software application has been developed by the HERA Team for calculating daily average for each station. The daily average depths of the water table are plotted against date to show the change in the depth of the water table at each station. CURRENT MONITORING RESULTS The monitoring results from the monitoring network have shown the changes in the depth of the water table at all stations (Fig. 4). The data obtained from field activities at Gibson Park and Hagan Stone Park stations indicated that the transducers for these two stations have been functioning properly. It is interesting that the depth of the water table has changed in a similar pattern and its fluctuation range is very similar at both stations for the monitoring period. At Gibson Park station, the water table showed an overall decreasing trend from April 2003 to September 2006 with an obvious seasonal variation. The water table rebounded approximately 8 feet at Gibson Park station and 5.5 feet at Hagan Stone Park from September 2006 to February The results are similar at Fire Station 37 and Prison Farm stations. At these two stations, a 15-foot range transducer (WL model) was installed initially. Because the water table fluctuated beyond instrument s detection range, it was replaced with a 30-foot range transducer (WL model). The records from both stations were inconsistent indicating that neither transducer worked properly (Fig. 4). In November 2006, both stations were upgraded with a WL16S-030 model transducer. The records obtained from February 2007 monitoring event indicated this new transducer functions properly. 9

9 Fig. 4 Change in the depth of the water table of Guilford County Monitoring network (vertical scales are different from station to station) 10

10 The depth of the water table at Monticello School was not recorded for some time periods because of a battery problem. Between May 2004 and May 2005, the groundwater table dropped approximately 5.5 feet but stayed relatively stable with fluctuation less than a foot since January At Fire Station 45, an initially installed 15-foot range transducer was replaced with a 30- foot range one because abnormal results were observed. It is interesting that the pattern of the water level change is similar to that at Gibson Park and Hagan Stone Park stations, particularly since September Two stations located in the northwestern part of the county are Triad Park station in Kernersville and Jack Dent Park station in Summerfield (Fig. 2). The water table at these two stations is deeper than those of other stations of the network (Fig. 4). The depth of the water table at Jack Dent Park is more than 60 feet below the top of casing during the monitoring period. A 15-foot range transducer was installed at both stations. The results from these stations indicate that the transducers have been working properly. Between May 2003 and March 2004, the data logger did not record the depth of the water table at Triad Park station because of probe or battery problems. Generally, this station has given good records since then. The depth of the water table at Triad Park station dropped almost in a linear pattern with very little fluctuation through October The water table has shown an overall decreasing trend with seasonal variations at Jack Dent station and has risen since October 2006 at both stations. The depth of the water table has fluctuated within 5 to 6 feet at both stations. Overall, the change in depth of groundwater table at all stations has been observed. The change is more significant at stations installed in the southern, and eastern parts of the county. The groundwater level at all stations has risen since October 2006 (Fig. 4). DISCUSSION Change in the depth of the water table with seasonal variation has been observed across the monitoring network. The hydrogeologic units underlying Guilford County may be the main factor affecting the variations in the depth of water. The northwestern part of the county is underlain mostly by an igneous, felsic intrusive hydrogeologic unit (Fig. 1). Because this unit tends to weather deeply and the weathering product from this unit is a deep sandy, porous regolith with high infiltration capacity, the recharge rate in this part of the county is higher (Daniel and Harned, 1988). More groundwater stores in relatively porous thick regolith and moves to bedrock in shorter time periods. Therefore, the seasonal change in the depth of the water table at Triad Park and Jack Dent Park is not significant (Fig. 4). Meta-igneous and meta-volcanic rocks underlie the central, southern and eastern parts of the county. The hydrogeologic units in these areas are highly metamorphic and fractured. The regolith from these units contains fine sand, silt and clay with moderate to low infiltration capacity. Therefore, the recharge rates are lower than that in northwestern area (Daniel and 11

11 Harned, 1988). The fracture remnants in the regolith provide conduits for surface water to migrate to saturated zones. At Prison Farm station and Fire Station 37, the change in the depth of the water table is irregular during the monitoring period. Bedrock is 12 feet below ground surface. Water from each precipitation event may move to the bedrock within a short time period. If future monitoring results from WL16S transducer show the same phenomenon, the irregularity of the change in water level at this station could be related to the shallow bedrock in the area. Precipitation may also affect the change in groundwater levels across the network. The high water level at all stations since October 2006 may reflect climate impact on the change in depth of the water table and probably some precipitation events across the county. It appears that the transducers may have produced some false records due to the instability of the instruments, climate, and/or batteries for the data logger. Assessment of equipment will be addressed when more monitoring events are conducted. CONCLUSION and FUTURE ACTIVITIES Change in the depth of the water table at all stations of the network has been observed. Seasonal variation are obvious at stations installed at Gibson Park, Hagan Stone Park, Fire Station 37 and 45, Prison Farm, and Monticello School. It appears that the hydrogeologic unit of the station may play a major role in this change. Future monitoring activities include: 1. Assess the stability of the transducers. Replace unstable WL15 transducers with WL16S-030, if necessary. 2. Establish a bi-monthly monitoring event to eliminate battery problems. 3. Obtain precipitation data. ACKNOWLEDGEMENT D&Y Well Drilling, Inc., McCall Brothers, Inc., Jones Well & Pump Company, Simmons Well Drillers, Inc., Built-Rite Well Drilling Company, Yadkin Well Company, Inc., and W.W. Maness and Sons, Inc. donated their well construction services for this project. Guilford County Department of Public Health sincerely appreciates their well construction services. Without their support, the groundwater network project is impossible. Thanks also go to Stephen Webb for helping initial station setup. 12

12 REFERENCE Daniel, C. C. and D. Harned, 1998, Ground-water Recharge to and Storage in the Regolithfractured Crystalline Rock Aquifer System, Guilford County, NC, USGS Water- Resources Investigation Report Daniel, C. C and R.A. Payne, 1990, Hydrogeologic unit map of the Piedmont and Blue Ridge Provinces of North Carolina, U.S. Geological Survey Water-Resources Investigations Report Floyd, E.O. and R.R. Peace, 1974, An appraisal of the ground-water resources of the Upper Cape Fear River Basin, North Carolina, North Carolina Office of Water and Air Resources Ground-Water Bulletin 20, 17 p. Kopec, R.J. and J. W. Clay, 1975, Climate and air quality, in Carolina atlas, portraits of a changing Southern States, Chapel Hill, The University of North Carolina Press, p Mundorff, M.J., 1984, Geology and groundwater in the Greensboro area, North Carolina: North Carolina Department of Conservation and Development Bulletin 55, p. 108 Spruill, T. B., and others, 1997, Radon in Groundwater in Guilford County, North Carolina, USGS FS , 4 p. 13