What is Spray Irrigation?

Spray irrigation is a method for disposing of secondary treated 

municipal wastewater by spraying it on the land surface (fig. 1). 

The sprayed wastewater either evaporates into the air, soaks into 

the soil, or percolates through the soil and recharges the ground 

water. 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 (Pennsylvania Department of Environmental Protection, 2003). The removal of nutrients is one advantage spray irrigation has to conventional 

disposal methods like instream discharge.

Why was Spray Irrigation Studied?

Spray irrigation was studied to determine if the treatment 

method increased recharge in the watershed and increased base 

flow in the stream draining the watershed where it was taking 

place and, if so, by how much. If recharge and base flow are 

increasing, the effects of spray-irrigated wastewater on groundand surface-water quality need to be determined. A nitrogen budget, and the role the plants and soils had in the nitrogen budget 

were determined to address the question, “What happens to the 

additional nitrogen added to the watershed from the wastewater?” A detailed description of the study design, methods, and 

results is presented in Schreffler and others (2005).

How was Spray Irrigation Studied?

The study was done in a 38-acre watershed in New Garden 

Township, Chester County, Pa. 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 the 

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. The 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 (fig. 2). 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 

bulk precipitation sampler to measure wet (rainfall or snow) and 

dry deposition of nitrogen was not installed on the site until 

August 1999. 

In cooperation with the Chester County Water Resources Authority 

Effects of Spray-Irrigated Municipal Wastewater on a Small 

Watershed in Chester County, Pennsylvania

by Curtis L. Schreffler and Daniel G. Galeone

U.S. Department of the Interior

U.S. Geological Survey Printed on recycled paper

USGS Fact Sheet FS–2005-3092

September 2005

Figure 1. View of 

spray head at the 

New Garden 

Township site, 

Chester County, 

Pennsylvania. 

Figure 2. The 38-acre watershed, 20-acre subbasin, spray fields, 

berm, flume, and stream gage at the New Garden Township site, 

Chester County, Pennsylvania.
pygg

Base Flow?

On an annual basis, spray irrigation increased the groundwater recharge to the study watershed. In order to determine the 

amount of increased recharge, water budgets for the study watershed were compared to water budgets of the Red Clay Creek 

Basin above the U.S. Geological Survey streamflow-gaging station (01479820) near Kennett Square, Pa. (drainage area 

28.3 mi2). The 38-acre (0.07 mi2) study watershed is within the 

larger Red Clay Creek Basin. The water budgets for Red Clay 

Creek would not show measurable affects of the spray irrigation 

because the amount of wastewater applied is small when compared to the larger size basin. Water budgets and recharge estimates were determined for 2000 and 2001 in the study watershed 

(fig. 3A and 3B) and Red Clay Creek Basin (table 1).

Table 1. Annual water budgets and recharge estimates for the Red 

Clay Creek Basin, 2000 and 2001.

[All units in inches]

Year Precipitation

Streamflow

Change in 

groundwater 

storage

EvapotranspirationGroundwater 

recharge

The total amount of ground-water recharge to the study watershed was 21.3 in. (inches) and 10.9 in. for 2000 (fig. 3A) and 

2001 (fig. 3B), respectively. The total recharge includes the natural recharge from precipitation plus additional recharge from 

spray irrigation. Compared to the annual recharge determined for 

the Red Clay Creek, spray irrigation increased recharge in the 

study watershed by approximately 8.8 and 4.3 in. for 2000 and 

2001, respectively. This was a 70-percent increase in recharge in 

2000 and a 65-percent increase in recharge in 2001 in the study 

watershed. In 2000, a total of 22.6 in. of wastewater was applied 

to the watershed and the 8.8 in. increase in recharge was equal to 

about 39 percent of the spray irrigation applied. In 2001, a total 

of 14.4 in. of wastewater was applied, and the increase in 

recharge of 4.3 in. was equal to about 30 percent of the spray irrigation applied.

Prior to wastewater application, base flow in the stream draining the watershed was determined for a 1-year period. The sprayirrigated wastewater increased base flow from the watershed. The 

magnitude of the increase was related to the time of year when 

established during the permitting process for the facility, and 

more wastewater was applied during the summer months than 

during the winter months. During the late fall through winter and 

into the early spring, when spray application rates were low, base 

flow increased by approximately 50 percent over the period prior 

to wastewater application. During the early spring through 

summer and into the late fall, when spray application rates were 

high, base flow increased by approximately 200 percent over the 

period prior to wastewater application. 

Did the Wastewater Affect Water Quality?

The spray-irrigated wastewater affected the ground-water 

quality in the watershed but did not affect the quality of stream 

water leaving the watershed at the streamflow-gaging station. 

The effects of the spray irrigation on ground-water quality were 

manifest in changes in concentrations of nitrate and chloride. 

Nitrate and chloride are in wastewater, and chloride is a by-product of the disinfection process at the sewage-treatment facility. 

Other chemical constituents were affected by the wastewater 

application, but the changes in concentrations were not as pronounced as nitrate and chloride. 

Nitrate and chloride have regulatory limits set by the U.S. 

Environmental Protection Agency for public drinking water supplies that are called maximum contaminant levels (MCLs) and 

secondary maximum contaminant levels (SMCLs). If the concentration of a constituent is above the MCL in a public water supply, some type of treatment to lower the concentration to a concentration less than the MCL is required. The MCL for nitrate is 

10 mg/L (milligrams per liter). SMCLs are based on aesthetic 

properties of water such as taste, odor, or the staining of plumbing fixtures. The SMCL for chloride is 250 mg/L. 

Effects on Ground-Water Quality

Overall, the spray-irrigated wastewater increased the concentrations of nitrate and chloride in ground water under the spray 

fields. The increase in concentrations appeared to take place after 

the spray irrigation began (fig. 4). However, a different pattern 

was observed in ground water at the bottom of the hill. There, the 

spray-irrigated wastewater decreased concentrations of nitrate 

and increased concentrations of chloride. The decreasing concentration of nitrate and the increasing concentration of chloride 

appeared to take place after the start of the spray irrigation 

(fig. 5). In particular, chloride concentrations appeared to 

increase about 1 year after the spray irrigation began. This was 

2000 42.9 19.4 0 23.6 12.5

2001 36.3 14.7 -2.7 24.3 6.6

Figure 3. Water budgets and recharge estimates for (A) 2000 

and (B) 2001 at the 

New Garden Township site, Chester 

County, Pennsylvania. 

(Water volume expressed as inches 

over the watershed 

area.)
probably because of the traveltime needed for the wastewater to 

reach the bottom of the hill.

Prior to the start of spray irrigation, nitrate concentrations in 

the ground water at the bottom of the hill near the pond were 

about 25 mg/L. These concentrations were higher than the average nitrate concentration in the wastewater. In this area, mushroom soil had been composted in the past (prior to 1990), and 

nitrate leached from the mushroom soil into the ground water. 

The net effect was that spray-irrigated wastewater with lower 

concentrations of nitrate (average 6.0 mg/L) flushed the ground 

water with higher concentrations of nitrate (25 mg/L) from the 

area, thus lowering the nitrate concentrations. 

Chloride is a conservative constituent. Chloride does not 

react, change, or transform within the environment and is carried 

readily with the water. Concentrations of chloride in ground 

water under the spray fields and in ground water at the bottom of 

the hill increased. In ground water under the spray fields, chloride 

concentrations increased throughout the study and approached 

the average concentration in the applied wastewater (fig. 4). 

Because the concentration of chloride in the wastewater was 

higher than the concentration in ground water at the bottom of the 

hill, chloride concentrations started to increase about 1-year after 

spraying began and continued to increase throughout the remaining study period (fig. 5).

Concentrations of nitrate in ground water under the spray 

fields increased throughout the study; however, the maximum 

4 monitor wells on the spray area was 3.7 mg/L. This concentration was below the MCL for nitrate and below the average nitrate 

concentration in the wastewater (fig. 4). The average concentration of nitrate in 33 wastewater samples was 6 mg/L. Nitrate is a 

non-conservative constituent in the environment. Nitrate can 

transform and is used by plants and micro-organisms and, as a 

result, its concentration can and does change. Nitrate concentrations in ground water under the spray fields increased but did not 

reach the average concentration in the wastewater, which indicated that some of the nitrate was being removed or transformed.

Effects on Stream-Water Quality Leaving the Watershed

As of the end of this study in December 2001, the spray-irrigated wastewater did not increase or decrease concentrations of 

nitrate or chloride in water from the pond or in stream base flow 

leaving the watershed. However, the wastewater affected the 

ground water under the spray fields and at the bottom of the hill. 

Because base flow is ground water discharged to streams, 

changes in the concentrations of nitrate and chloride are expected 

sometime in the future, but when this will take place is unknown. 

Changes in stormflow or storm-related loadings of nitrate and 

chloride in the stream were not assessed.

What Happened to the Nitrogen?

The combination of inputs, outputs, and changes in storage of 

nitrogen (in pounds) is referred to as a nitrogen budget. The nitrogen budget for the 20-acre subbasin was determined from June 

1999 through December 2001, when all inputs, outputs, and storage changes of nitrogen were determined or estimated. Input 

sources of nitrogen to the 20-acre subbasin included the spray 

wastewater and wet and dry deposition from the atmosphere. In 

addition, certain plants called legumes take nitrogen out of the air 

and deliver it to the soil. This could be an input of nitrogen to the 

basin. Output sources of nitrogen included stormflow and base 

flow leaving the subbasin at the flume (fig. 2), ammonium degassing from the wastewater being sprayed (volatilization), harvesting and removing the crop, and ground-water migration out of the 

basin. In addition, denitrification, a process in which nitrogen is 

converted to nitrogen gas that dissipates into the atmosphere, 

could be a possible output. The storage of nitrogen in the subbasin was in the soil, the water in the soil, and the ground water 

under the site. 

In nature, nitrogen is comprised of different forms such as 

nitrate, nitrite, ammonium, and organic nitrogen. For this factsheet, only the results for total nitrogen are discussed. Information about the types and forms of nitrogen at this site is presented 

in Schreffler and others (2005). 

For the study period June 1999 through December 2001, a 

total of approximately 5,420 lb (pounds) of nitrogen was added 

to the subbasin from the wastewater (fig. 6). The wastewater was 

the largest contributor of nitrogen to the subbasin. About 

75 percent or 4,100 lb of nitrogen from the wastewater was 

applied during the growing season, April through September. 

The other input of nitrogen was from the atmosphere as wet (rainfall or snow) and dry deposition. Inputs from atmospheric deposition were distributed relatively evenly throughout the year. The 

total amount of nitrogen from the atmosphere was 380 lb of wet 

and 110 lb of dry deposition measured from August 1999 through 

December 2001. An additional 90 lb of nitrogen was estimated to 

Figure 5. Maximum nitrate and chloride concentrations in water from 

two monitor wells at the bottom of the hill at the New Garden Township site, Chester County, Pennsylvania.

Figure 4. Maximum nitrate and chloride concentrations in water from 

four monitor wells in the spray fields and average wastewater concentrations at the New Garden Township site, Chester County, Pennsylvania. 
be deposited in June and July 1999 for a total of 580 lb over the 

June 1999 through December 2001 study period (fig. 6). Input 

from legumes was assumed to be zero because a uniform cover 

crop of orchardgrass, which is not a legume, was planted in the 

spray fields. 

The primary nitrogen output or loss from the 20-acre subbasin 

was plant harvesting. Plant harvesting removed about 4,560 lb of 

nitrogen during the three growing seasons from 1999 to 2001 or 

about 77 percent of the total nitrogen removed during the study 

period. An additional 1,060 lb of nitrogen was removed from the 

watershed in ground-water migration off site. This accounted for 

18 percent of the total nitrogen removed from the 20-acre subbasin. Total nitrogen leaving the subbasin in streamflow was 

250 lb, which accounted for about 4 percent of the total nitrogen 

removed from the subbasin. In addition, approximately 60 lb of 

nitrogen (1 percent of the total) was lost through ammonia volatilization. This took place only during the growing season when 

air temperatures were high. 

The changes in storage of nitrogen in soil, the water in the soil, 

and ground water were quantified. Additional details on the complexities of the different forms of nitrogen are presented in Schreffler and others (2005). In summary, the amount of nitrogen 

stored in the soil was the predominant storage component for 

nitrogen in the 20-acre subbasin. Some forms of nitrogen 

increased in the soil from spring 1999 to fall 2001. However, concentrations of nitrate in the soil generally showed no change over 

the study period. 

Nitrogen stored in soil water and ground water substantially 

decreased over the study period in the 20-acre subbasin. The 

amount of nitrogen in the soil water and ground water in springsummer 1999 was about twice as much as the amount of nitrogen 

in water collected in the last samples in 2001. Looking at the 20-

acre subbasin as a whole, the amount of nitrogen in ground water 

was reduced even though concentrations of nitrate in water samples from monitor wells on the spray area showed significant 

increases during the study period. However, because the sprayirrigated water helped to flush nitrate from the mushroom compost disposal area out of the 20-acre subbasin, the amount of 

stored nitrogen in the ground-water system as a whole decreased. 

Overall, the change in storage of nitrogen in soil, the water in the 

soil, and ground water was a net loss of about 1,370 lb. 

Based on the estimates for the individual components of the 

nitrogen budget, the amount of nitrogen input and released from 

storage exceeded the amount of nitrogen lost from the basin by 

or underestimating outputs (or a combination of both) and 

reflects the difficulty in accurately quantifying all terms in the 

nitrogen budget. 

Conclusions

Wastewater has to be treated and eventually disposed. Land 

disposal of wastewater through spray irrigation at this site 

increased recharge to the watershed and increased the base flow 

leaving the watershed. As a result of spray irrigation, concentrations of nitrate and chloride increased in ground water under the 

spray fields. Although nitrate concentrations were increasing, 

they remained below the average concentration of nitrate in the 

applied wastewater, which indicated that some of the nitrate 

being applied was being removed. Also, the nitrate concentrations were below the MCL set by the USEPA for nitrate. Chloride 

concentrations were increasing in ground water under the spray 

fields and were approaching the average concentration of chloride in the wastewater. The chloride concentrations were below 

the SMCL set by the USEPA for chloride. Increasing chloride 

concentrations indicate that wastewater is reaching the ground 

water and other contaminants or constituents that may be present 

in the wastewater also could be reaching the ground water. 

The nitrogen budget indicated spray irrigation did not cause an 

increase in nitrogen losses in ground and surface waters leaving 

the 20-acre subbasin from June 1999 through December 2001. In 

addition, no net increase in the storage of nitrogen in the form 

used by plants and micro-organisms took place in soil and ground 

water under the 20-acre subbasin. Plant uptake and harvesting 

was the primary route for removing nitrogen from the 20-acre 

subbasin. Seventy-five percent of the total nitrogen in the sprayirrigated wastewater was applied during April through October. 

This spray-irrigation site was designed so that some nitrogen 

applied would be removed from the site through harvesting of 

plant material and that did take place. In fact, some nitrogen 

already stored in the soil before spray irrigation started was 

removed from the subbasin through harvesting of the crop. These 

data show the importance of plant harvesting at spray-irrigation 

sites and the importance of timing spraying with plant growth so 

that much of the applied nitrogen is used by plants. The results of 

the study have shown that the landscape can be used to filter 

nitrogen out of the water prior to discharge from the watershed, 

and at this site, spray irrigation was a better method of disposing 

of wastewater from the treatment facility than discharging the 

water directly to a stream. 

References Cited

Pennsylvania Department of Environmental Protection, 2003, Water quality antidegradation implementation guidance: Bureau of Water Supply and Wastewater Management, Document Number 391-0300-002. 

Schreffler, C.L., Galeone, D.G., Veneziale, J.M., Olsen, 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, 158 p.

Figure 6. Nitrogen budget June 1999 through December 2001 at the 

New Garden Township site, Chester County, Pennsylvania. 

The authors would like to thank: 

Pennsylvania Department of Environmental Protection

New Garden Township Board of Supervisors, and 

New Garden Township Sewer Authority

Chester County Health Department