Abstract

Shallow wells from rangelands on a south Florida flatwoods soil were sampled between 1993 and 1995 to monitor water quality parameters (N and P). Objectives were to investigate nutrient concentrations and water table depths, evaluate natural levels of phosphorus in ground water relative to state regulations, and project the potential for off-site discharges of phosphorus in surface and subsurface flow. The monitoring stations consisted of three shallow ground water wells (2-4 m deep) at a slough, a freshwater pond, and a flatwoods range site. Water table depth measurements and duplicate water samples were taken monthly. Ground water samples were analyzed for 10 constituents, pH, and electrical conductivity. Nitrate levels for all three range sites tended to be zero except for an occasional spike measuring less than 1 mg N/L. Ammonia concentrations fluctuated between 0 and 2 mg N/L. Total phosphorus concentrations for all three wells tended to be below the regulatory level of 0.18 mg P/L and averaged less than half this amount. For low-relief flatwoods sites, ground water is a reasonable indicator of surface and subsurface off-site discharges. Thus, unfertilized rangelands would be expected to comply with surface runoff standards for similar areas in Florida.

Introduction

Native range is a valuable natural resource in Florida, especially with respect to the beef industry. Forage production from rangeland is utilized by grazing cattle during the winter season (November-March). Unlike improved pasture, rangeland is extensively managed with no fertilization. Information is lacking concerning water quality from unfertilized rangeland in Florida. The objectives of this study were to monitor water quality parameters from unfertilized rangeland and compare results with state regulatory limits governing off-site discharges of phosphorus in some portions of south Florida.

Phosphorus is the primary factor controlling the eutrophication of many surface waters and is thought to limit the productivity of Lake Okeechobee. Significant sources of P contributions to the lake are dairy and beef operations. Major upgrading of stormwater and milking parlor wash-water treatment practices has already been required of dairy farmers in south Florida. In an effort to finally reach the target reductions for P loading to Lake Okeechobee, regional authorities are contemplating requiring similar improvement in beef cattle operations.

Methods

Water samples were collected monthly from three unfertilized Florida range sites (freshwater pond, south Florida flatwoods, and slough) from January 1993 until December 1995 at the Southwest Florida Research and Education Center near Immokalee, Florida (Figure 1). Three ground water wells, one at each range site, were installed so that the well screen was located 3 m below the soil surface. Soils at each site were Alfisols (nearly level, somewhat poorly drained sandy soils with a clay horizon) or Spodosols (nearly level, somewhat poorly drained sandy soils with a spodic horizon 1-2 m below the soil surface). The ground water well at the freshwater pond site was located on the periphery near the pond's edge (Figure 2).

The flatwoods site (Figure 3) is more of an upland environment characterized by somewhat drier conditions. The slough site (Figure 4) is characterized by intermittent wet conditions. The study site (40 ha) had not been grazed or fertilized in the last 40 years. Prior to collecting water samples, water table depth was recorded and approximately three well volumes of water was removed. Duplicate water samples (500 ml) were collected monthly from each well, stored in ice, transported to the laboratory, and analyzed for pH and electrical conductivity prior to filtering. Samples were filtered and approximately 25 ml of water was analyzed for nitrate, ammonium, total phosphorus and several other constituents (K, Cl, Na, Ca, Mg, Fe, Mn, Zn, and Cu).

Results

Depth to the water table fluctuates through the year with the shallowest depths normally occurring in the summer (June to September) and the greatest depth typically in the winter and spring. However, the winter of 1994-1995 was extremely wet and high water conditions persisting for most of the winter (Figure 5). Table 1 presents results (averages) of ground water sample analyses. Water pH over all three sites ranged from 4.2-5.9. These pH values are characteristic of the acid soils in south Florida.

Ammonia concentrations (Figure 6) averaged less than 0.1 mg N/L with occasional peaks approaching 0.4 mg N/L. Ammonification of organic nitrogen occurs near the surface under aerobic conditions and percolates to the water table with rainfall. Almost no nitrate N was detected in the water during the study period (Figure 7). Absence of nitrate N could have resulted from de-nitrification in these shallow ground water aquifers or lack of nitrification because soil pH was below 4.5 and little ammonia was available. Even on fertilized pastures in south Florida, significant concentrations of nitrate are not typically observed in ground water.

Total P levels from all three sites were variable during the sampling period (Figure 8). High water table conditions in the wet summer months could reduce P concentrations because of dilution. Accordingly, the trend of increasing rainfall over the period of record was accompanied by a general decrease in P levels. Regardless, no relationship of P to water table depth was noted. On a very few occasions, P levels exceeded maximum limits set by the South Florida Water Management District for surface water runoff from rangeland (0.18 mg/L). Despite occasional peaks, the yearly sample mean for P for all three sites did not exceed the SFWMD limit of 0.18 mg/L. However, yearly mean total P levels for the freshwater pond were twice that of the slough and flatwoods site. The freshwater pond collects and filters nutrients that accumulate from upland discharge, therefore, it is not surprising that total P levels were elevated.

Discussion

Little ammonia or nitrate was detected in the shallow ground water of native south Florida rangelands. The characteristic high-water table, acidic soil conditions of Florida's flatwoods rangelands appear to prevent significant accumulation or offsite discharges of nitrogen as either ammonia or nitrate.

Total P levels from all three sites were variable, though total P concentrations at some of the sampling periods exceeded the SFWMD limit (0.18 mg/l) for total P in surface runoff from rangeland. The fact that average P concentrations in ground water from ungrazed rangeland averaged approximately a third the regulatory limit and occasionally exceeded the limit suggests that grazing of south Florida rangelands potentially could result in off-site phosphorus discharges in excess of regulatory limits.

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