Vulnerability of a Public Supply Well in a Karstic Aquifer to Contamination

Abstract

To assess the vulnerability of ground water to contamination in the karstic Upper Floridan aquifer (UFA), age-dating tracers and selected anthropogenic and naturally occurring compounds were analyzed in multiple water samples from a public supply well (PSW) near Tampa, Florida. Samples also were collected from 28 monitoring wells in the UFA and the overlying surficial aquifer system (SAS) and intermediate confining unit located within the contributing recharge area to the PSW. Age tracer and geochemical data from the earlier stage of the study (2003 through 2005) were combined with new data (2006) on concentrations of sulfur hexafluoride (SF(6)), tritium ((3)H), and helium-3, which were consistent with binary mixtures of water for the PSW dominated by young water (less than 7 years). Water samples from the SAS also indicated mostly young water (less than 7 years); however, most water samples from monitoring wells in the UFA had lower SF(6) and (3)H concentrations than the PSW and SAS, indicating mixtures containing high proportions of older water (more than 60 years). Vulnerability of the PSW to contamination was indicated by predominantly young water and elevated nitrate-N and volatile organic compound concentrations that were similar to those in the SAS. Elevated arsenic (As) concentrations (3 to 19 microg/L) and higher As(V)/As(III) ratios in the PSW than in water from UFA monitoring wells indicate that oxic water from the SAS likely mobilizes As from pyrite in the UFA matrix. Young water found in the PSW also was present in UFA monitoring wells that tap a highly transmissive zone (43- to 53-m depth) in the UFA.

Effectiveness of Monitored Natural Attenuation (MNA) as a Groundwater Remedy for Arsenic in Phosphatic Wastes

“…Redox conditions are also important for understanding occurrences of elevated arsenic concentrations in the lower intermediate aquifer. Previous studies have attributed relatively high arsenic concentrations to the infiltration of surface water and subsequent oxidation of pyrite in these formations (Katz et al ; Katz et al ; Ayotte et al ). Although the present model predicted that surface water would have reached lower intermediate aquifer wells by the early 2000s (~28–31 m; Figure C), oxygen was predicted to have been consumed prior to this depth.…”

“…The highest groundwater arsenic concentrations historically occurred in the surficial aquifer beneath the Main Plant Area and the primary process water pond (P1) (Figure ), with concentrations predominantly greater than 0.01 mg/L (the EPA Maximum Contaminant Level, or MCL) at these locations. In deeper groundwater, arsenic concentrations have generally been within the expected natural background range (0.001–0.02 mg/L) (Katz et al , ; Ayotte et al ).…”

“…The reversal in vertical groundwater flow directions resulted in the migration of more oxidizing water into the subsurface, which has led to the partial dissolution of naturally occurring pyritic minerals. Pyrite dissolution releases iron to form additional iron oxyhydroxide mineral coatings; however, some arsenic can also be released in the process, resulting in locally elevated arsenic concentrations in groundwater (Katz et al ; Ayotte et al ).…”

TracerLPM (Version 1): An Excel® workbook for interpreting groundwater age distributions from environmental tracer data

“…A public-supply well located within the Rio Grande principal aquifer in Albuquerque, New Mexico, was studied as part of the NAWQA program's "Transport of Anthropogenic and Natural Contaminants (TANC) to supply wells" topical team (Bexfield and others, 2012). Samples for 3 H, 14 C, and chlorofluorocarbons (CFCs) were collected in June and December 2007, November 2008, and May 2009 The data in this report are from the studied supply well or SSW in Bexfield and others (2012). All 14 C measurements were corrected for dilution from non-radioactive ("dead") carbon sources by using PHREEQC and NetpathXL (Parkhurst and Appelo, 1999;Plummer and others, 1994).…”

The Pollutant Mechanism of Water Supply in Henan Oilfield

“…The situation is heavier and heavier, which is concerned by many scholars [1][2][3], especially in the oilfield [4].…”