USE OF ISOTOPES, AGE-DATING, AND NUMERICAL SIMULATION TO EVALUATE SOURCE HISTORIES AND TRANSPORT OF NO₃ TO PUBLIC SUPPLY WELLS IN PRINCIPAL AQUIFERS OF THE UNITED STATES

 

Peter Mcmahon¹, Johnkarl Böhlke², Craig Brown³, Karen Burow⁴, Christy Crandall⁵, Matthew Landon⁶

 

¹U.S. Geological Survey, Lakewood, Colorado, USA; email: pmcmahon@usgs.gov

²U.S. Geological Survey, Reston, Virginia, USA

³U.S. Geological Survey, Hartford, Connecticut, USA

⁴U.S. Geological Survey, Sacramento, California USA

⁵U.S. Geological Survey, Tallahassee, Florida, USA

⁶U.S. Geological Survey, Lincoln, Nebraska USA

 

6th International Symposium on Applied Isotope Geochemistry, September 11-16, 2005, Prague, Czech Republic

 

Introduction

Nitrate is one of the most common contaminants in drinking water produced by public-supply wells in the United States (U.S. EPA, 2001).  For example, in the State of California, about 10 percent of public-supply wells produce water that exceeds the drinking-water standard for NO₃ of 714 μM (10 mg/L as N) (Esser et al., 2002).  The U.S. Geological Survey began a study in 2001 to better understand source histories and transport of nitrate and other natural and anthropogenic contaminants to public-supply wells in selected principal aquifers of the U.S. (Fig. 1) (Eberts, 2005).

 

Source Histories of NO₃

 Isotopes of NO₃ (N,O) and N₂, concentrations of NO₃ and dissolved gases (N₂, Ar, Ne), and age-dating (CFCs, SF₆, ³H-³He) were used to correct for denitrification effects and to reconstruct records of NO₃ concentration and δ¹⁵N values in recharge to representative public supply wells in the Central Valley aquifer (CVA) in California, High Plains aquifer (HPA) in Nebraska, Glacial Deposit aquifer (GDA) in Connecticut, and the Floridan aquifer (FA) in Florida.  Nitrate concentration and δ¹⁵N values in recharge to the CVA and HPA exhibited large increases during the last 50 years, coincident with the growth in N applications in these agriculture-dominated areas.  Nitrate concentration and δ¹⁵N values in recharge to the GDA and FA were relatively stable during the last 15 to 30 years in these areas dominated by residential, commercial, and forested land uses.  Median NO₃ concentrations in recharge were larger in the CVA and HPA (237 to 254 μM) than in the GDA and FA (<35 to 103 μM).  The large increase in NO₃ concentrations in recharge with time in the CVA and HPA indicates that NO₃ concentrations in public-supply wells screened in those aquifers will increase in the future as progressively more recent recharge is transported to the wells unless recharge conditions change or processes in the aquifer attenuate NO₃ concentrations.

 

Transport of NO₃

Nitrate transport in the four principal aquifers is affected, to varying degrees, by numerous factors that include source history, denitrification, well construction, and well operation.  For example, denitrification in fine-grained sediments overlying producing zones in the HPA and FA effectively removes NO₃ from ground water, as indicated by decreasing NO₃ concentrations, fractionation of isotopes in NO₃ and N₂ (Fig. 2), and accumulation of excess N₂ in ground water along flow paths to the wells.  Denitrification is of lesser importance in the CVA and GDA.  The effectiveness of denitrifying zones in the HPA and FA is reduced in places by well screens that cross denitrifying zones and by well pumping that increases ground-water velocities, thus reducing reaction times.  Although well characteristics such as these can result in increased NO₃ concentrations in produced water, they also can be actively managed to potentially reduce NO₃ concentrations in produced water.

 

Conclusions

The combined use of isotopes, age-dating, and numerical simulation provides new insights into the factors controlling NO₃ transport to representative public-supply wells in principal aquifers of the U.S.  Nitrate concentrations in water from the studied public-supply wells are controlled by multiple factors that include NO₃ source history (related to land use), biogeochemical processes in the aquifer (such as denitrification), and well characteristics.

 

Figure 1. Map showing the location of selected principal aquifers in the United States in which studies of contaminant transport to public-supply wells are occurring.

 

Figure 2. Nitrogen isotope composition of residual NO₃ and excess N₂ as a function of denitrification reaction progress in the Central Valley aquifer (CVA), High Plains aquifer (HPA), Floridan aquifer (FA), and Glacial Deposits aquifer (GDA).

 

References

Eberts S.M., Erwin M.L., and Hamilton P.A. (2005): Assessing the vulnerability of public-supply wells to contamination from sources in the environment. U.S. Geological Survey Fact Sheet 2005-3022, 4 p.

 

Esser B., Hudson B., Moran J. (2002): Nitrate contamination in California groundwater: An integrated approach to basin assessment and resource protection. Lawrence Livermore National Laboratory UCRL-ID-151454 DRAFT, 62 p.

 

U.S. Environmental Protection Agency (2001): Factoids: Drinking water and ground-water statistics for 2000. EPA 816-K-01-004, 12 p.