POTENTIAL TRANSPORT OF THE HERBICIDE MSMA AND ARSENATE (+5) FROM GOLF COURSES TO GROUNDWATER IN SOUTHEASTERN FLORIDA
Monosodium methanearsonate (MSMA) is an arsenical organic compound, extremely soluble, that is frequently used as a herbicide for the postemergent control of grassy weeds in turf grasses at golf courses in southeastern Florida. Typical application rates range from 0.45 to 1.96 lb of
MSMA/acre. Recent characterizations undergone in Dade County, Florida, by the Florida Department of Environmental Protection (FDEP) found dissolved arsenic concentrations up to 12 mg/L in samples taken from golf course monitoring wells. These wells monitor local water quality in the
unconfined Biscayne aquifer, which is the sole source of drinking water supply in the county. The FDEP has set maximum contaminant levels for groundwater quality at 50 μg/L.
This study examined the potential susceptibility of the local shallow groundwater to arsenic contamination
from the use of MSMA, with normal application rates, at a local golf course. Soil samples were taken from the West Palm Beach Country Club, located in southeastern Florida, to conduct batch type adsorption experiments for determining the sorptivity (Kd) of MSMA and its expected
inorganic byproduct, arsenate As (+5), to local soils. Background characterization of 4 distinct soil horizons, from which 5 representative samples were taken at even intervals, indicated non-detectable levels of Total Arsenic in the soil column down to the water table (approximately
8.5 feet below grade at boring site). Each horizon consisted predominantly (99% by weight) of sands, with phosphorus (P) and iron (Fe) concentrations increasing with depth. The soil samples provided a background experimental reference, showing not detectable levels of either naturally
occurring or anthropogenic arsenic.
Previous findings, which were experimentally confirmed by this study, reported that most inorganic arsenic (As) species need approximately 48 hours to reach equilibrium between the adsorbed phase and the dissolved phase. Batch reactor tests of this study
yielded data to select best sorption isotherms, for both MSMA and As (+5), with the 5 different soil samples, which represented 5 vertical horizons. Results indicated that all contaminant/soil systems displayed a linear sorptivity relationship (R2 ranging from 0.8262 to
0.9779) with Kd values (units of L/g) ranging from 0.0102 to 0.2295. Results also showed that As(+5) had a higher affinity for the 5 different soil fractions than MSMA.
Utilizing laboratory derived Kd values, as well as available physical, chemical, and daily
meteorological data, modeling of the MSMA/As(+5) soil systems was performed to illustrate the potential fate and transport of MSMA at golf courses in southeastern Florida. Simulations of contaminant movement in the unsaturated zone were conducted with a U.S. Environmental Protection
Agency (USEPA) documented model, Pesticide Root Zone Model 2.0 (PRZM 2.0). PRZM 2.0 is a one-dimensional, dynamic, compartmental model that is able to estimate chemical movement in unsaturated soil systems, within and immediately below the plant root zone. Eight scenarios were evaluated to
illustrate the effect of varying field conditions such as irrigation, dispersion, and Kd values. In each of the eight scenarios there was transport of both MSMA and arsenate, which resulted in: (1) significant migration of MSMA and As(+5) out of the vadose zone and into the
water table within 1 month from only 1 application; and (2) Total Dissolved As concentrations were persistently found at the water table interface that exceeded 50 μg/L and ranged from 0.0 to 400 μg/L.
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