Skip Links

USGS - science for a changing world

Scientific Investigations Report 2010–5099

Prepared in cooperation with
City of Tallahassee

Nitrate-N Movement in Groundwater from the Land Application of Treated Municipal Wastewater and Other Sources in the Wakulla Springs Springshed, Leon and Wakulla Counties, Florida, 1966–2018

By J. Hal Davis, Brian G. Katz, and Dale W. Griffin

Thumbnail image of report cover

ABSTRACT

The City of Tallahassee began a pilot study in 1966 at the Southwest Farm sprayfield to determine whether disposal of treated municipal wastewater using center pivot irrigation techniques to uptake nitrate-nitrogen (nitrate-N) is feasible. Based on the early success of this project, a new, larger Southeast Farm sprayfield was opened in November 1980. However, a recent 2002 study indicated that nitrate-N from these operations may be moving through the Upper Floridan aquifer to Wakulla Springs, thus causing nitrate-N concentrations to increase in the spring water. The increase in nitrate-N combined with the generally clear spring water and abundant sunshine may be encouraging invasive plant species growth. Determining the link between the nitrate-N application at the sprayfields and increased nitrate-N levels is complicated because there are other sources of nitrate-N in the Wakulla Springs springshed, including atmospheric deposition, onsite sewage disposal systems, disposal of biosolids by land spreading, creeks discharging into sinks, domestic fertilizer application, and livestock wastes.

Groundwater flow and fate and transport modeling were conducted to simulate the effect of all of the nitrate-N sources on Wakulla Springs from January 1, 1966, through December 31, 2018. The total simulated nitrate-N load to Wakulla Springs in 1967 was a relatively modest 69,000 kilograms per year (kg/yr). The major sources of the nitrate-N load in 1967 were determined to be:

1.   Inflow to the study area across the lateral model boundaries at 31,000 kg/yr (45 percent),
2.   Biosolids disposal by land spreading at 14,000 kg/yr (20 percent),
3.   Creeks discharging into sinks at 7,800 kg/yr (11 percent), and
4.   The Southwest Farm sprayfield at 4,500 kg/yr (7 percent).

The total simulated nitrate-N load to Wakulla Springs in 1987 had increased dramatically to 297,000 kg/yr. The major sources of nitrate-N load in 1987 were determined to be:

1.   The Southeast Farm sprayfield at 186,000 kg/yr (63 percent),
2.   Biosolids at 37,000 kg/yr (12 percent), and
3.   Inflow to the study area across the lateral model boundaries at 36,000 at kg/yr (12 percent). All of the other sources were 5 percent or less.

The Wakulla Springs discharge can change rapidly, even during periods of little or no rainfall. This rapid change is probably the result of Wakulla Springs intermittently capturing groundwater that has been going to the Spring Creek Springs Group. This spring group is located in a marine estuary and is affected by tidally influenced saltwater intrusion. Two modeling scenarios were simulated and results are presented for 2007 and 2018 in an effort to bracket the range of possible current and future changes in the flow of Wakulla Springs. In scenario 1, it was assumed that Wakulla Springs was not capturing Spring Creek Springs Group flow. In scenario 2, it was assumed that Wakulla Springs was capturing Spring Creek Springs Group flow.

Under the assumptions of scenario 1, the total simulated nitrate-N load to Wakulla Springs in 2007 was 207,200 kg/yr. The major sources of nitrate-N load were determined to be:

1.   The Southeast Farm sprayfield at 111,000 kg/yr 53 percent),
2.   Inflow to the study area across the lateral model boundaries at 44,000 at kg/yr (21 percent), and
3.   Onsite sewage disposal systems at 24,000 kg/yr (12 percent).

All of the other sources contributed 6 percent or less. Under the assumptions of scenario 2, the total simulated nitrate-N load to Wakulla Springs was 294,000 kg/yr. The major sources of nitrate-N load were determined to be:

1.   The Southeast Farm sprayfield at 111,000 kg/yr (38 percent),
2.   Onsite sewage disposal systems at 56,000 kg/yr (19 percent),
3.   Inflow to the study area across the lateral model boundaries at 52,000 at kg/yr (18 percent), and
4.   Creeks discharging into sinks at 31,000 kg/yr (11 percent).
All of the other sources contributed 8 percent or less.

The nitrate-N loads to Wakulla Springs from the Southeast Farm sprayfield for scenarios 1 and 2 were both 111,000 kg/yr. These amounts were the same because most of the water from the Southeast Farm sprayfield went into Wakulla Springs in both simulations. In contrast, the nitrate-N loads from onsite sewage disposal systems for scenarios 1 and 2 were 24,000 kg/yr and 56,000 kg/yr, respectively. The additional water captured by Wakulla Springs in scenario 2 came from an area that had a high density of residential and commercial sites using onsite sewage disposal systems

Under the assumptions of scenario 1, the total simulated nitrate-N load to Wakulla Springs in 2018 will be 156,000 kg/yr. The major sources of nitrate-N load for scenario 1 are anticipated to be:

1.   Inflow to the study area across the lateral model boundaries at 48,000 at kg/yr (31 percent),
2.   The Southeast Farm sprayfield at 42,000 kg/yr (27 percent),
3.   Onsite sewage disposal systems at 32,000 kg/yr (21 percent), and
4.   Fertilizer at 17,000 kg/yr (11 percent).

All of the other sources will contribute 5 percent or less. Under the assumptions of scenario 2, the total simulated nitrate-N load to Wakulla Springs in 2018 will be 266,000 kg/yr. The major sources of nitrate-N load for scenario 2 are anticipated to be:

1.   Onsite sewage disposal systems at 80,000 kg/yr (30 percent),
2.   Inflow to the study area across the lateral model boundaries at 57,000 at kg/yr (21 percent),
3.   The Southeast Farm sprayfield at 43,000 kg/yr (16 percent),
4.   Creeks discharging into sinks at 31,000 kg/yr (12 percent), and
5.   Fertilizer at 32,000 kg/yr (12 percent).
All of the other sources will contribute 6 percent or less.

The simulated nitrate-N load from the Southeast Farm sprayfield to Wakulla Springs during 2007 through 2018 decreases from 111,000 kg/yr to 42,000 kg/yr in scenario 1 and decreases from 111,000 kg/yr to 43,000 kg/yr in scenario 2. Both scenarios show these decreases because of the simulated planned reduction in the concentration of nitrate-N in the wastewater used for irrigation from approximately 12 milligrams per liter (mg/L) in 2007 to 3 mg/L in 2018. In contrast, the simulated nitrate-N load from onsite sewage disposal systems to Wakulla Springs from 2007 through 2018 increases from 24,000 kg/yr to 32,000 kg/yr in scenario 1, and increases from 56,000 kg/yr to 80,000 kg/yr in scenario 2. Both scenarios show increases respective to the increases in population and residential and commercial sites using onsite sewage disposal systems. In addition, the simulated nitrate-N load to Wakulla Springs from 2007 through 2018 from inflow to the study area across the lateral model boundaries increases from 44,000 kg/yr to 48,000 kg/yr in scenario 1, and increases from 54,000 kg/yr to 57,000 kg/yr in scenario 2. Both scenarios show increases due to increasing nitrate-N levels upgradient in Leon County.

Revised, March 2, 2011

First posted June 16, 2010

For additional information contact:
Hal Davis, Hydrologist
U.S Geological Survey
Suite A200
2639 North Monroe St.
Tallahassee, FL 32303
850-553-3673
http://fl.water.usgs.gov/

Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.


Suggested citation:

Davis, J.H., Katz, B.G., and Griffin, D.W., 2010, Nitrate-N movement in groundwater from the land application of treated municipal wastewater and other sources in the Wakulla Springs springshed, Leon and Wakulla Counties, Florida, 1966-2018: U.S. Geological Survey Scientific Investigations Report 2010-5099, 90 p.



Contents

Abstract

Introduction

Purpose and Scope

Previous Investigations

Description of the Study Area

Background and Approach

Geohydrologic Setting of the Wakulla Springs Springshed

Geologic Setting

Hydrologic Setting

Groundwater Flow

Data Collection and Field Methods

Well and Core Samples

Nitrate-N Loading and Concentrations at Land Surface from Various Sources

Southeast and Southwest Sprayfields

Atmospheric Deposition

Effluent Discharges from Onsite Sewage Disposal Systems

Disposal of Biosolids by Land Spreading

Creeks Discharging into Sinks

Fertilizer Application

Livestock Wastes

Nitrate-N and Chloride Concentrations in the Upper Floridan Aquifer and Wakulla Springs

Model Development

Groundwater Flow Model Description and Calibration

Subregional Model Geometry

Boundary Conditions

Simulated Hydraulic Conductivities

Simulated Recharge to the Upper Floridan Aquifer

Subregional Model Calibration

Simulated Effective Porosity

Fate and Transport Model and Calibration

Hydrodynamic Dispersion

Simulation of Nitrate-N and Chloride Concentrations from Various Sources

Southeast and Southwest Farm Sprayfields

Effluent Discharges from Onsite Sewage Disposal Systems, Fertilizer Application, and Livestock Wastes

Inflow at Model Boundaries

Disposal of Biosolids by Land Spreading

Creeks Discharging into Sinks and Atmospheric Deposition

Nitrate-N and Chloride Concentrations in Wakulla Springs

Simulated Future Nitrate-N Concentrations in Wakulla Springs

Simulated Nitrate-N Concentration Distribution in the Upper Floridan Aquifer at Selected Times

End of 1967

End of 1986

End of 2004

End of 2006

End of 2007

End of 2018

Simulated Nitrate-N Loading to the Upper Floridan Aquifer

Simulated Nitrate-N Loading to Wakulla Springs from All Sources

Southeast and Southwest Farm Sprayfields

Atmospheric Deposition

Effluent Discharges from Onsite Sewage Disposal Systems

Inflow at Model Boundaries

Disposal of Biosolids by Land Spreading

Creeks Discharging into Sinks

Fertilizer Application

Livestock Wastes

Model Sensitivity Analysis

Groundwater Flow Model Sensitivity Analysis

Fate and Transport Model Sensitivity Analysis

Model Limitations

Summary

References


Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2010/5099/index.html
Page Contact Information: USGS Publications Team
Page Last Modified: Thursday, 10-Jan-2013 19:11:22 EST