Using Multiple Chemical Indicators to Assess Sources of Nitrate and Age of Groundwater in a Karstic Spring Basin

Abstract

Human health and ecological concerns have arisen due to a steady increase in nitrate-N concentrations during the past 40 years in Fannin Springs (0.3-4.7 mg/L), a regional discharge point with an average flow of >2.8 m 3 /second (>100 ft 3 /second) for water from the karstic Upper Floridan aquifer (UFA). Multiple chemical indicators (major dissolved species, 15 N and 18 O of nitrate, dissolved gases, 78 pesticides and degradates, and 67 organic compounds typically found in domestic and industrial wastewater) and transient tracers ( 3 H/ 3 He, chlorofluorocarbons [CFCs], sulfur hexafluoride [SF 6 ]) were analyzed in water samples from nine wells along three transects and in spring water to assess groundwater age and potential contaminant sources. Land use is predominantly agricultural (52 percent) and forest (31 percent) in the 320 km 2 (124 mi 2 ) spring basin, which was delineated from a potentiometric-surface map of the UFA using high-resolution water-level data. Nitrate-N concentrations were highly variable in the oxic UFA and ranged from <0.02 to 4.7 mg/L. d 15 N-NO 3 values (3.4-9.9 per mil) indicated that nitrate contamination originated from inorganic sources (synthetic fertilizer) and organic sources (manure spreading or waste disposal). Higher nitrate concentrations and the younger age of spring water relative to water from upgradient wells indicate better communication with N sources at the surface. Apparent ages of groundwater correlated positively with well depth (P < 0.05) and were younger in water from wells nearer to the spring (<8 years) compared with other wells (10-50 years). Most transient tracer concentrations were consistent with binary mixing curves representing mixtures of water recharged during the past 10 years and older water (recharged before 1940). Young water mixing fractions ranged from 0.07 to 0.90. Trace levels of herbicides found in groundwater and spring water were indicative of applications for vegetative control in agricultural and other land-use types.

Groundwater geochemistry of the Chihuahua City region in the Rio Conchos Basin (northern Mexico) and implications for water resources management

“…Szabo et al (1996) observed that CFCs may be affected by local, near-surface sources such as septic tanks and other sources associated with urban development. It also is apparent that CFC-11 values are depleted in comparison with CFC-12 and CFC-113, which might be due to a shorter lifetime and faster degradation in the soil and groundwater zone as illustrated in various previous studies (Tesoriero et al, 2000;Böhlke et al, 2002;Lindsey et al, 2003;Katz et al, 2005;Happell et al, 2006). Large time lags through diffusion processes are expected if the transport of trace gases occurs through thick unsaturated zones (Cook and Solomon, 1995), especially when the water table is >50 m below ground level.…”

“…Alternatively, geochemical and isotopic investigations have been demonstrated to be very useful tools in assessing groundwater flow components such as recharge area, flow paths, flow velocity and discharge areas, in other semiarid and arid areas (Edmunds et al, 2002;Mahlknecht et al, 2004Mahlknecht et al, , 2006aPortugal et al, 2005). Apart from radiogenic isotopes, chlorofluorocarbons (CFCs) have proven to be feasible tracers for groundwater residence times, recharge estimation and contribution of recent water (Busenberg and Plummer, 1992;Eckwurzel et al, 1994;Reilly et al, 1994;Oster et al, 1996;Plummer and Busenberg, 2000;Katz et al, 2005;Mahlknecht et al, 2006b). These tracers are not commonly employed as residence time indicators in deeper aquifers due to diffusion processes in the large unsaturated zone (Cook and Solomon, 1995;Engesgaard et al, 2004).…”

“…However, the presence of these gases in groundwater indicates recent anthropogenic impact. Waste dumps are frequently a source of freons (Oster et al, 1996;Katz et al, 2005).…”

Hydrologic and biotic influences on nitrate removal in a subtropical spring‐fed river

“…Since the mid-20th century, NO { 3 concentrations in Florida springs have risen more than an order of magnitude from historic concentrations of approximately 0.05 mg N L 21 to greater than 1 mg N L 21 or higher in many springs (Scott et al 2004;Upchurch et al 2007). Isotopic signatures and mass balance models indicate that agricultural sources (principally fertilizer, but also animal waste) are primarily responsible for increased N loads (Katz et al 2005). Elevated NO { 3 concentrations have been linked to the proliferation of algal blooms in springs (Stevenson et al 2007, but see Heffernan et al [in press] for a critical review of evidence supporting that link), and spring discharge accounts for a significant proportion of the N load that reaches coastal oceans via the Suwannee and St. Johns Rivers (Pittman et al 1997;Cohen 2008;Magley and Joyner 2008).…”

Direct and Indirect Effects of Organic Matter Sources on Denitrificaton in Florida Rivers

“…As in rivers throughout the world, nitrate concentrations have increased in many northern Florida spring-fed and blackwater rivers over the last several decades (Bledsoe and Phlips 2000, Scott et al 2004. Agricultural inputs, including synthetic fertilizer application and livestock waste are thought to be the main source of enrichment (Katz et al 2005). High NO 3 -inputs to these ecosystems may lead to algae blooms as a result of eutrophication, degrading habitat quality (Vitousek et al 1997, Galloway et al 2003, although other factors may be primarily responsible for algal blooms in springs (Heffernan et al 2010b).…”