Effects of Spray-Irrigated Municipal Wastewater on a Small Watershed
in Chester County, Pennsylvania
This project was done in cooperation with the
Chester County Water Resources Authority, the
Pennsylvania Department of Environmental Protection, and
New Garden Township.
INTRODUCTION
An increasing number of communities in Pennsylvania are implementing
land-treatment systems to dispose of treated sewage effluent.
Spray irrigation is a method for disposing of secondary treated municipal
wastewater by spraying it on the land surface. The sprayed wastewater
either evaporates into the air, soaks into the soil, or percolates through the
soil and recharges the ground-water system. Land application of wastewater has
advantages over conventional means of disposal by direct discharge to streams
because the wastewater recharges the ground-water system and increases base flow
in streams. Additional benefits are derived from the "natural" treatment of the
wastewater that takes place in the soil when plants and other biota remove some
nutrients (nitrogen and phosphorus) from the wastewater. The removal
of nutrients is one advantage
spray irrigation has to conventional disposal methods like instream discharge.
The U.S. Geological Survey (USGS), in cooperation with the Pennsylvania Department of Environmental
Protection, Chester County Water Resources Authority, and the New Garden
Township Sewer Authority, conducted a study from October
1997 through December 2001 to assess the effects of spray irrigating secondary
treated sewage effluent on a small watershed
in New Garden Township, Chester County, Pa.
THE STUDY SITE
The study was done in a 38-acre watershed in New Garden Township.
Ground and surface water, soil, soil water, precipitation,
wastewater, and plant material were all sampled in the study. Streamflow-gaging
stations, monitor wells, soil probes, and a weather station were installed in
the watershed, and the data from these instruments were used to answer
recharge, base-flow, and water-quality questions. Streamflow, ground-water
levels, and water-quality data were collected from May 1998 through December
2001. The soil probes and the weather station were installed in May 1999.
A nitrogen budget was determined for a smaller 20-acre subbasin of the watershed
for the period June 1999 through December 2001. The 20-acre subbasin included
the fields where the spray irrigation was being applied and the area directly
downhill from the fields. A berm was constructed to capture all
stormflow runoff from the spray fields due to precipitation and direct it
through a flume, which is a structure in which flow is determined from the
height of the water passing through. Soil-water instruments and a bulk
precipitation sampler also were installed at the study site, and the data were
used to explain what happened to the nitrogen.
The study determined the
effects the spray-irrigated effluent had on ground-water and surface-water
quantity and quality of the small watershed and assessed the fate and transport
of Nitrogen (N). Annual and monthly water budgets were determined and N loading fate and
transport were tracked. In the annual and monthly water budgets,
evapotranspiration and recharge estimates
were determined and evaluated.
STUDY CONCLUSIONS
On an annual basis, 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 also 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.
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 application, 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.
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 f
actor 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.
STUDY REPORTS
Schreffler, C.L., Galeone, D.G., Veneziale, J.M., Olson, L.E., and O'Brien, D.L., 2005,
Effects of Spray-Irrigated Treated Effluent on Water Quantity and Quality, and the
Fate and Transport of Nitrogen in a Small Watershed, New Garden Township, Chester County,
Pennsylvania: U.S. Geological Survey Scientific Investigations Report 2005-5043.
[ on-line ]
[ PDF file ]
Schreffler, C.L., Galeone, D.G., 2005, Effects of Spray-Irrigated Municipal Wastewater on a Small Watershed in
Chester County, Pennsylvania: U.S. Geological Survey Fact Sheet 2005-3092.
[ on-line ]
[ PDF file ]
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