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| Pennsylvania Water Science Center |
U.S. Geological Survey Scientific Investigations Report 2005-5043
By Curtis L. Schreffler, Daniel G. Galeone, John M. Veneziale, Leif E. Olson, and David L. O'Brien
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An increasing number of communities in Pennsylvania are implementing land-treatment systems to dispose of treated sewage effluent. Disposal of treated effluent by spraying onto the land surface, instead of discharging to streams, may recharge the ground-water system and reduce degradation of stream-water quality. The U.S. Geological Survey (USGS), in cooperation with the Pennsylvania Department of Environmental Protection (PaDEP) and the Chester County Water Resources Authority (CCWRA) and with assistance from the New Garden Township Sewer Authority, conducted a study from October 1997 through December 2001 to assess the effects of spray irrigation of secondary treated sewage effluent on the water quantity and quality and the fate and transport of nitrogen in a 38-acre watershed in New Garden Township, Chester County, Pa.
On an annual basis, the spray irrigation increased the recharge to the watershed. Compared to the annual recharge determined for the Red Clay Creek watershed above the USGS streamflow-gaging station (01479820) near Kennett Square, Pa., the spray irrigation increased annual recharge in the study watershed by approximately 8.8 in. (inches) in 2000 and 4.3 in. in 2001. For 2000 and 2001, the spray irrigation increased recharge 65-70 percent more than the recharge estimates determined for the Red Clay Creek watershed. The increased recharge was equal to 30-39 percent of the applied effluent.
The spray-irrigated effluent increased base flow in the watershed. The magnitude of the increase appeared to be related to the time of year when the application rates increased. During the late fall through winter and into the early spring period, when application rates were low, base flow increased by approximately 50 percent over the period prior to effluent application. During the early spring through summer to the late fall period, when application rates were high, base flow increased by approximately 200 percent over the period prior to effluent application.
The spray-irrigated effluent affected the ground-water quality of the shallow aquifer differently on the hilltop and hillside topographic settings of the watershed where spray irrigation was being applied (application area). Concentrations of nitrate-nitrogen (nitrate N) and chloride (Cl) in the effluent were higher than concentrations of these constituents in shallow ground water from wells on the hilltop and hillside prior to start of spray irrigation. In water from wells on the hilltop, concentrations of nitrate N and Cl increased in samples collected during effluent application compared to samples collected prior to effluent application. Also, increasing trends in concentration of these two constituents were evident through the study period. In water from wells on the hillside, which were on the eastern part of the application area, nitrate N and Cl concentrations increased in samples collected during effluent application compared to samples collected prior to effluent application. Also, increasing trends in concentration of these two constituents were evident through the study period. However, on the hillside of the western application area, the ground-water quality was not affected by the spray-irrigated effluent because of the greater thickness of unconsolidated material and higher amounts of clay present in those unconsolidated sands. Although nitrate N concentrations increased in water from hilltop and hillside wells in the application area, the nitrate N concentrations were below the effluent concentration. A combination of plant uptake, biological activity, and denitrification may be the processes accounting for the lower nitrate N concentrations in shallow ground water compared to the spray-irrigated effluent. Cl concentrations in water from hilltop western application area well Ch-5173 increased during the study period but were an order of magnitude less than the input effluent concentration. Cl concentrations in shallow ground water in the eastern application area approached the median chloride concentration in the spray-irrigated effluent of 90 mg/L (milligrams per liter). The chloride concentrations in water from wells Ch-5180 and Ch-5179 were 74 and 61 mg/L in samples collected in December 2001, which was when data collection ended.
The spray-irrigated effluent affected the ground-water quality of the shallow aquifer in the valley bottom, which was outside the application area. Nitrate N concentrations were lower and Cl concentrations were higher in the effluent than concentrations of these constituents in shallow ground water in the valley bottom because of past land-use practices. Historically, spent mushroom substrate was disposed of in this area. The spent mushroom substrate leached nitrate N into the shallow aquifer causing elevated concentrations of nitrate N (>25 mg/L). In water in the valley bottom, nitrate N concentrations decreased and chloride concentrations increased when comparing samples collected prior to application to samples collected during effluent application. The increased hydraulic loading of spray-irrigated effluent flushed out the higher concentrated nitrate N water from the area. Cl concentrations started to increase after approximately 1 year of effluent being applied, which may be due to lag time of the effluent water reaching the valley bottom.
Spray-irrigated effluent did affect ground water in the bedrock aquifer on the hilltop application area and in the valley bottom but ground water in the bedrock aquifer on the hillside application areas was not affected. Concentrations of nitrate N and Cl increased slightly in water from wells on the hilltop probably because vertical downward head (water-level) differences between the shallow and bedrock aquifers were greatest on the hilltop. The overall effect in the valley bottom was the dilution of higher concentrations of nitrate N, Cl and other constituents present in the in-situ ground water because of the increased hydraulic loading.
As of the end of this investigation in December 2001, the spray-irrigated effluent did not affect the water quality of the pond or the stream base flow leaving the watershed with respect to concentrations of nitrate N. Stormflow or loadings were not assessed. However, because the shallow aquifer under the application area was affected, the water quality of the pond and stream base flow will most likely be affected sometime in the future, but the timing can not be determined.
The effects of effluent application on N fate and transport were studied in a 20-acre subbasin within the 38-acre watershed. Possible N inputs to the system include atmospheric deposition, effluent spray irrigation, and N fixation by leguminous plants. Possible N outputs include loss through volatilization of ammonia in spray water during irrigation, denitrification processes in subsurface zones, water discharge from the subbasin, and plant uptake and subsequent removal during harvest. Changes in N storage can occur in the soil matrix, both in the solid and liquid phase, and in the ground-water system.
N inputs to the 20-acre subbasin from June 1999 through December 2001 averaged about 190 lb (pounds) per month; about 91 percent of this was input from spray-irrigated effluent and the remaining was from precipitation events. Approximately 70 percent of the 5,420 lb of N applied in effluent from June 1999 through December 2001 was inorganic N. Measured atmospheric deposition of N from August 1999 through December 2001 was 490 lb. The forms of N in atmospheric deposition were almost equally distributed between nitrate N (36 percent), organic N (33 percent), and ammonia N (30 percent). Inputs from precipitation were distributed relatively evenly throughout the year; spray-irrigation inputs were highest during the growing season (75 percent of the spray-irrigated effluent was applied from April through September). It was assumed that N fixation by microorganisms was zero over the study period.
The primary N output from the 20-acre subbasin was from plant harvesting. Plant harvesting removed about 4,560 lb of N during the three growing seasons from 1999 to 2001 or about 77 percent of the total N output during the study period. Assuming that only the inorganic-N portion of the spray-irrigated effluent was available to plants, the additional N taken up by plants was from the store of N in the soil matrix. These data indicate the importance of plant harvesting at spray-irrigation sites and the importance of timing applications with plant growth so that some of the applied N is recovered by plants.
Water discharge and ammonia volatilization accounted for the remaining 23 percent of the N output from the 20-acre subbasin. Water discharge from the subbasin occurred as underflow (beneath a swale) and water captured by the swale and discharged from the subbasin through a flume. N output from underflow accounted for about 18 percent (or about 1,060 lb) of the total N output from the 20-acre subbasin. Approximately 94 percent of the dissolved N leaving the 20-acre subbasin in underflow was in the form of nitrate with the remaining fraction organic N. Water discharge through the flume accounted for about 4 percent (or about 250 lb of N) of the total N output from the 20-acre subbasin. The primary forms of N in water discharged through the flume were organic N (57 percent) and nitrate N (36 percent). Ammonia volatilization was another seasonally dependent component that was found to occur only during the growing season when air temperatures were higher than during the rest of the year. Loss of N through ammonia volatilization was estimated to be about 60 lb during the study period.
N stored in the solid-soil phase was the predominant form of N in the 20-acre subbasin. The average amount of N in the solid-soil phase over the entire 20-acre subbasin for soil depths of 0-4 ft was 170,000 lb. Approximately 98-99 percent of N in the 0-4 ft depth interval was in organic form. Inorganic forms of N in the solid-soil phase from 0 to 4 ft indicated an increase from spring 1999 to fall 2001. The mass of ammonium ions increased from approximately 700 lb in spring 1999 to 1,600 lb in fall 2001 (at depths from 0-4 ft). Concentrations of nitrate ions in the solid-soil phase basically indicated no change over the same period. Unlike nitrate, which is transported through the soil system relatively rapidly, ammonium ions are retained in the soil.
N stored in the soil water and shallow ground water substantially decreased over the study period in the 20-acre subbasin. The mass of N in the soil water and shallow ground-water compartments in spring-summer 1999 was about twice as much as the mass of N for the last samples collected in 2001. Approximately 86-87 percent of N in soil water and ground water to the depth of competent bedrock was in the form of nitrate N. The mass of N in shallow ground water was reduced even though shallow wells at the top of the 20-acre subbasin indicated significant increases in concentrations of nitrate N during the study period. Effluent application helped to flush the soil nitrate from the spent mushroom substrate out of the system, thus decreasing the mass of stored N in the shallow aquifer.
The N balance for the site indicated that spray irrigation did not cause any increasing trend in N losses in water discharging from the 20-acre subbasin from June 1999 through December 2001. There was also no net increase in the storage of inorganic N in subsurface compartments. Plant uptake of N appeared to be the primary factor in minimizing the loss of N from the 20-acre subbasin. Seventy-five percent of the N load from spray-irrigated effluent was applied from April through October. This spray site was designed so that some N applied during effluent application would eventually be removed from the site through harvesting of plant material.
Abstract
Introduction
Purpose and Scope
Previous Investigations
Description of Study Area
Climate
Drainage
Soils
Hydrogeology
Vegetation
Historical Land Use
Standard Facility Operations
Methods of Investigation
Assessing Effects of Spray-Irrigated Effluent on Water Quantity
Monthly Water Budget
Precipitation
Applied Effluent
Streamflow
Ground-Water Underflow
Pond Storage
Ground-Water Storage
Soil-Moisture Storage
Calibration of Soil-Moisture Probes
Volumetric Unsaturated Soil-Water-Storage Determination
Crop-Referenced Evapotranspiration
Annual Water Budget
Recharge
Assessing Effects of Spray-Irrigated Effluent on Water Quality
Assessing Fate and Transport of Nitrogen
Seasonal Nitrogen Budget
Atmospheric Deposition
Effluent Nitrogen Input
Solid-Soil Nitrogen Storage
Soil-Water Nitrogen Storage
Shallow Ground-Water Nitrogen Storage
Ammonia Volatilization
Plant Removal of Nitrogen
Discharge
Underflow
Quality Control
Samples for the Water-Quality Assessment
Samples for the Nitrogen Fate and Transport Assessment
Effects of Spray-Irrigated Effluent on Water Quantity
Annual Water Budget
Monthly Water Budget
Recharge
Base Flow
Evaluation of the Effects on Water Quantity
Effects of Spray-Irrigated Effluent on Water Quality
Effluent Quality
Ground Water
Shallow Aquifer
Physical Properties and Chemical Constituents Measured in the Field \
Nutrients
Major and Minor Ions
Metals and Other Trace Constituents
Bedrock Aquifer
Physical Properties and Chemical Constituents Measured in the Field \
Nutrients
Major and Minor Ions
Metals and Other Trace Constituents
Surface Water
Nutrients
Major and Minor Ions
Metals and Other Trace Constituents
Evaluation of the Effects on Water Quality
Fate and Transport of Nitrogen
Atmospheric Deposition
Effluent Nitrogen Input
Solid-Soil Nitrogen Storage
Soil-Water Nitrogen Storage
Shallow Ground-Water Nitrogen Storage
Volatilization
Plant Removal of Nitrogen
Discharge
Underflow
Evaluation of the Fate and Transport of Nitrogen
Summary
Acknowledgments
References Cited
Glossary
Appendix 1. Results of chemical and physical analyses of effluent, wet and dry deposition, soil, pan, and plant samples collected at the New Garden Township spray-irrigation site, Chester County, Pennsylvania, 1999-2001
This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.
View the full report in PDF 6.3 MB
For more information about USGS activities in Pennsylvania contact:
Director
USGS Pennsylvania Water Science Center
215 Limekiln Road
New Cumberland, Pennsylvania 17070
Telephone: (717) 730-6960
Fax: (717) 730-6997
or access the USGS Water Resources of Pennsylvania home page at:
http://pa.water.usgs.gov/.
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