Lake Wales Ridge Groundwater:
Overview of Agricultural Chemicals:Pesticides and Nitrate
Concentrations of agricultural chemicals in ground water underlying citrus groves on Lake Wales Ridge are indicative of the extreme vulnerability of this region to leaching from the land surface into ground water. This overview includes summaries of concentrations and detection rates in ground water from the network wells over time, as well as comparisons with human-health guidance levels for drinking water.
Several factors should be considered in evaluating these results and their implications regarding drinking-water supplies and human health effects. The wells tap the surficial aquifer, which although used for some private drinking-water supply wells, is not typically used for municipal drinking-water supply. Although rates of detection for some chemicals have been fairly high in a spatial context (expressed as a percentage of all network wells), exceedances of Florida’s human-health guidance concentrations for drinking water for any single pesticide compound during the 4-year sampling period have been limited to a maximum of about 6% of all samples. These differences between detection rates and health-guidance exceedances occur due to fluctuations of chemical concentrations in ground water over time and to the use of laboratory detection limits that are lower than health-guidance concentrations. Exceedances of health-guidance values for nitrate (as N) over time have been higher (about 61% of all samples) than those for pesticides, and active research and implementation of fertilizer “Best Management Practices” are being focused on the Ridge to reduce leaching of nitrate into ground water. Additionally, specific restrictionsa apply to the application of some agricultural chemicals (see federal [ link to: http://www.nasda-hq.org/nasda/nasda/Foundation/state/Federal.pdf] and state [link to: http://www.nasda-hq.org/nasda/nasda/Foundation/state/Florida.pdf] environmental laws affecting agriculture). Because some of these restrictions apply only to areas surrounding potable (drinking-water supply) wells, chemical concentrations in ground water from monitoring wells, such as those used in this study, may be greater than concentrations in the vicinity of potable wells. Detection frequencies should always be evaluated in terms of the associated minimum detectable concentrations, which often vary between different agricultural chemicals and can vary according to the laboratory methods used for a specific study. Higher detection frequencies will typically occur when laboratory detection limits are lowered.
Based on Ridge sampling between 1999 and 2003, nitrate (as N) concentrations in ground water exceeded the drinking-water standard (maximum contaminant level) of 10 mg/L one or more times at 90% of the 31 network wells (Figure 3). The regional occurrence of pesticide and degradate detections of concentrations exceeding laboratory detection limits (note: not human-health guidance levels) in ground water at least once during this period included: norflurazon and desmethyl norflurazon (84% of wells), simazine (61%), bromacil (52%), diuron (52%), deisopropylatrazine (CEAT) (39%), aldicarb sulfoxide (32%), aldicarb sulfone (32%), metalaxyl (10%), aldicarb (6%), imidacloprid (6%), and thiazopyr monoacid (6%).
The distributions of agricultural chemical concentrations from the Lake Wales Ridge study are summarized in table 4. This summary includes comparisons with health-guidance concentrations and with results from USGS national sampling. Exceedances of Florida health-guidance concentrations for drinking water occurred for five target pesticides (or degradates) and two pesticide-degradate sums, and the frequency of exceedances for these compounds ranged from less than 1 percent to 5.6 percent of all samples. Concentrations of total pesticides, summed per sample, ranged from less than method detection limits to 225 µg/L, with a median value of 13 µg/L (n=384 samples).
Between 1999 and 2003, the total number of target pesticides and degradates detected per well ranged from no detections to a maximum of 8 different compounds. Eighty-one percent of the wells yielded detections of 3 or more different compounds during this period (Figure 4). Two of the 31 network wells have continually yielded ground water with no detectable concentrations of target pesticides, and one well yielded infrequent detections of a single target pesticide. There do not appear to be any strong spatial patterns of pesticide occurrence (Figure 6).
Maximum concentrations of all detected pesticides exceeded national maximums from a network of approximately 5,000 wells sampled as part of the USGS National Water Quality Assessment (NAWQA) Program (table 4). The NAWQA network included 1,443 wells in agricultural areas; 31 of these wells occurred in a flatwoods citrus study unit in southern Florida. Florida citrus groves in “ridge” compared to “flatwoods” regions differ in many respects, including a much lower propensity for leaching of agricultural chemicals in the flatwoods regions than in the ridge regions. Also, regarding national comparisons, it should be noted that usage patterns and application rates of pesticides vary regionally (for example, see http://ca.water.usgs.gov/pnsp/use92/index.html), and, for agricultural chemicals in ground water, maximum concentrations are typically much higher than more commonly occurring concentrations. The Ridge maximums for pesticides and degradates were generally at least 2 to 4 times higher than the 95% quantilesb for the Ridge concentrations, and were at least 2.5 to 440 times the median observed values (table 4).
Quarterly repeat sampling results indicate relative consistency in the suite of pesticides detected in a given well over time, but significant short-term variability. Nitrate concentrations frequently varied by more than 10 mg/L per quarter, but ranges in concentration have been relatively consistent for wells with long term records (Figure 7). Pesticide and degradate concentrations fluctuated above and below health-guidance concentrations at some wells (Figure 5). The observed temporal variability in this study indicates that repeat sampling is essential to adequately characterize the range of concentrations occurring in this region, and implies that such variation may also occur in other similar ground-water systems.
a Florida has adopted several rules designed to mitigate risks associated with pesticide use. Particularly relevant to the Lake Wales Ridge are 5E-2.028 Florida Administrative Code (FAC) and 5E-2.038 FAC. 5E-2.038 FAC prohibits the application of bromacil to non-bedded citrus grown on vulnerable soils and specifies the soil classifications where bromacil applications are prohibited. In addition, 5E-2.028 FAC prohibits aldicarb applications to citrus within 300 or 1000 feet of potable wells. The buffer size is dependent on the soil type at the site of application and the depth of the potable well, and is defined in 5E-2.028 FAC The regulations that establish these restrictions on bromacil and aldicarb use identify the soil classifications that are considered highly vulnerable with respect to ground-water contamination.
bQuantiles are values associated with specific exceedance probabilities; for example the 95% quantile values are likely to be exceeded 5% of the time, based on the data collected in this study.