Galveston Bay Atmospheric Deposition Studies
DateJan. 25, 2
Wade, TL; Sweet, ST
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In order to fulfill the mandates of the Great Waters Program and the Clean Air Act Amendments of 1990 (112 m), the US EPA initiated monitoring research in important and representative water bodies, including coastal waters. As part of this program, the Texas Regional Integrated Atmospheric Deposition Study (TRIADS) was established by GERG with a sampling site located in Seabrook, Texas in order to monitor atmospheric deposition of contaminants to Galveston Bay (Sweet et al., 1999). Wet depositional nutrient monitoring at the TRIADS site started on February 2, 1995 and was in continuous operation until August 6, 1996. Total nitrogen concentrations ranged from 7.72 to 365 Ýmol/L. The highest concentration was present predominantly as NO3 (357 Ýmol/L) The average rainfall during this study was 963 mm/year, approximately 25% below the average rainfall to Galveston Bay of approximately 1270 mm/ year. The yearly nitrogen deposition rate to Galveston Bay was estimated as 6.16 Kg/hectare-year. This number can be used in the model developed by Patwardhan and Donigian (1995) to provide an estimate of the nitrogen budget for Galveston Bay. This model assumes that wet deposition is equal to dry deposition so total deposition would be 12.2 kg/ hectare-year. The total input from atmospheric deposition of nutrient nitrogen directly to the Bay is estimated as 1.76x106 Kg/year or 8.6% of the total nutrient nitrogen input to Galveston Bay with another 2.8% from atmospheric input to the watershed. Therefore, atmospheric inputs supplies about 10% of the nutrient nitrogen to Galveston Bay in 1996. Estimates of the atmospheric deposition to Galveston Bay of polycyclic aromatic hydrocarbons (PAHs), PCBs and chlorinated pesticides (e.g., HCHs, chlordanes, and DDTs) were made using precipitation and meteorological data that were collected continuously from 2 February 1995 to 6 August 1996 at Seabrook, TX, USA (Park et al. 2001 a,b). Particulate and vapor phase PAHs in ambient air and particulate and dissolved phases in rain samples were collected and analyzed. More than 95% of atmospheric PAHs were in the vapor phase and about 73% of PAHs in the rain were in the dissolved phase. Total PAH concentrations ranged from 4 to 161 ng/m3 in air samples and from 50 to 312 ng/l in rain samples. Temporal variability in total PAH air concentrations were observed, with lower concentrations in the spring and fall (4-34 ng/m3) compared to the summer and winter (37-161 ng/m3). PAHs in the air near Galveston Bay are derived from both combustion and petroleum vaporization. Gas exchange from the atmosphere to the surface water is estimated to be the major deposition process for PAHs (1211 ug/m2y), relative to wet deposition (130 ug/m2y) and dry deposition (99 ug/m2y). Annual deposition of PAHs directly to Galveston Bay from the atmosphere is estimated as 2 t/yr. The direct deposition rate (wet + dry) of PCBs to Galveston Bay was 6.40 ug/m2y. The net flux from gas exchange estimated for PCBs was from Galveston Bay water to the atmosphere (78 ug/m2y) and was the dominant flux. These studies provide results consistent with other areas where more detailed studies have been undertaken (e.g. Chesapeake Bay). They were however in many cases limited in scope and duration (mostly due to funding constraints). Atmospheric deposition is seen to be an important process in the budget of contaminates for Texas Bays and estuaries, but needs to be more extensively studied.