Challenges Associated with Generating Reliable Nanogram-per-Liter Measurements of Hexavalent Chromium in Groundwater.

The US Environmental Protection Agency (USEPA) and several states are considering regulatory limits for hexavalent chromium (Cr(VI)) <100 ng/L, increasing the importance of obtaining accurate and reliable data. The present study presents the results of 2 studies investigating the effectiveness of chemical preservation on ng/L concentrations of Cr(VI) in groundwater. Chemically preserved and unpreserved aliquots of a groundwater sample were spiked with a known amount of Cr(VI), and then analyzed in tandem with sets of preserved and unpreserved performance test samples over 28 d. The studies showed that although Cr(VI) appears to be relatively stable over 28 d, it may be sensitive to minor matrix changes after collection. The studies also demonstrated significant challenges to measuring Cr(VI) below 100 ng/L, because the results can be subject to positive bias or contamination that may not be readily apparent. We conclude that Cr(VI) measurements using a USEPA published method below 100 ng/L may be categorized as semiquantitative at best. Environ Toxicol Chem 2020;39:1526-1533. © 2020 SETAC.

Pore Water Collection, Analysis and Evolution: The Need for Standardization.

Investigating the ecological impacts of contaminants released into the environment requires integration of multiple lines of evidence. Collection and analysis of interstitial water is an often-used line of evidence for developing benthic exposure estimates in aquatic ecosystems. It is a well-established principle that chemical and toxicity data on interstitial water samples should represent in-situ conditions; i.e., sample integrity must be maintained throughout the sample collection process to avoid alteration of the in-situ geochemical conditions. Unfortunately, collection and processing of pore water is not standardized to address possible geochemical transformations introduced by atmospheric exposure. Furthermore, there are no suitable benchmarks (ecological or human health) against which to evaluate adverse effects from chemicals in pore water; i.e., empirical data is lacking on the toxicity of inorganic contaminants in sediment interstitial water. It is clear that pore water data is best evaluated by considering the bioavailability of trace elements and the partitioning of contaminants between the aqueous and solid phases. It is also evident that there is a need for sediment researchers and regulatory agencies to collaborate in developing a standardized approach for sediment/pore water collection and data evaluation. Without such guidelines, the number of different pore water collection and extraction techniques will continue to expand, and investigators will continue to evaluate potentially questionable data by comparison to inappropriate criteria.

Synchrotron-based imaging of chromium and γ-H2AX immunostaining in the duodenum following repeated exposure to Cr(VI) in drinking water.

Current drinking water standards for chromium are for the combined total of both hexavalent and trivalent chromium (Cr(VI) and Cr(III)). However, recent studies have shown that Cr(III) is not carcinogenic to rodents, whereas mice chronically exposed to high levels of Cr(VI) developed duodenal tumors. These findings may suggest the need for environmental standards specific for Cr(VI). Whether the intestinal tumors arose through a mutagenic or non-mutagenic mode of action (MOA) greatly impacts how drinking water standards for Cr(VI) are derived. Herein, X-ray fluorescence (spectro)microscopy (µ-XRF) was used to image the Cr content in the villus and crypt regions of duodena from B6C3F1 mice exposed to 180 mg/l Cr(VI) in drinking water for 13 weeks. DNA damage was also assessed by γ-H2AX immunostaining. Exposure to Cr(VI) induced villus blunting and crypt hyperplasia in the duodenum--the latter evidenced by lengthening of the crypt compartment by ∼2-fold with a concomitant 1.5-fold increase in the number of crypt enterocytes. γ-H2AX immunostaining was elevated in villi, but not in the crypt compartment. µ-XRF maps revealed mean Cr levels >30 times higher in duodenal villi than crypt regions; mean Cr levels in crypt regions were only slightly above background signal. Despite the presence of Cr and elevated γ-H2AX immunoreactivity in villi, no aberrant foci indicative of transformation were evident. These findings do not support a MOA for intestinal carcinogenesis involving direct Cr-DNA interaction in intestinal stem cells, but rather support a non-mutagenic MOA involving chronic wounding of intestinal villi and crypt cell hyperplasia.

Natural attenuation of coal combustion waste in river sediments.

The weathering of coal combustion products (CCPs) in a lotic environment was assessed following the Tennessee Valley Authority (Kingston, TN) fly ash release of 2008 into surrounding rivers. Sampled materials included stockpiled ash and sediment collected from 180 to 880 days following the release. Total recoverable concentrations of heavy metals and metalloids in sediment were measured, and percent ash was estimated visually or quantified by particle counts. Arsenic and selenium in sediment were positively correlated with percent ash. For samples collected 180 days after the release, total concentrations of trace elements downstream of the release were greater than reference levels but less than concentrations measured in stockpiled ash. Total concentrations of trace elements remained elevated in ash-laden sediment after almost 2.5 years. A sequential extraction procedure (SEP) was used to speciate selected fractions of arsenic, copper, lead, nickel, and selenium in decreasing order of bioavailability. Concentrations of trace elements in sequentially extracted fractions were one to two orders of magnitude lower than total recoverable trace elements. The bulk of sequentially extractable trace elements was associated with iron-manganese oxides, the least bioavailable fraction of those measured. By 780 days, trace element concentrations in the SEP fractions approached reference concentrations in the more bioavailable water soluble, ion exchangeable, and carbonate-bound fractions. For each trace element, the percentage composition of the bioavailable fractions relative to the total concentration was calculated. These SEP indices were summed and shown to significantly decrease over time. These results document the natural attenuation of leachable trace elements in CCPs in river sediment as a result of the loss of bioavailable trace elements over time.

A performance-based framework for demonstrating appropriate use of an alternate method for wastewater compliance monitoring.

The choice of wastewater compliance methods used in the United States has been largely prescribed; however, in some cases, this has led to data of unknown or poor quality. This problem is further compounded by the relatively slow regulatory approval process to incorporate discharge-specific method modifications or flexibility to using alternate, potentially better technologies. In this study, a framework is presented, using a performance-based-system approach, which a discharger could use to verify proper use of an alternate or modified method. An example, using two chemical oxygen demand methods (a currently approved method and an alternate method that does not generate hazardous waste) demonstrates that the protocol is simple to use, yet scientifically defensible and effective and that this approach should be readily understandable to both regulators and the regulated community. Our results also suggest that the reference method approach, without associated measurement quality objectives, may yield a false sense of competency with an alternate method.

Determination of hexavalent chromium by using speciated isotope-dilution mass spectrometry after microwave speciated extraction of environmental and other solid materials.

Precise and accurate determination of hexavalent chromium in different types of solid environmental sample is regarded as a technical challenge with significant potential error if historically accepted methods are used. Microwave-assisted alkaline extraction (0.5 mol L(-1) NaOH + 0.28 mol L(-1) Na2CO3) followed by anion-exchange chromatographic separation and inductively coupled plasma mass spectrophotometric detection has been shown to provide accurate and precise results. To obtain a better understanding of potential species conversion during and/or after extraction steps, speciated isotope-dilution mass spectrometry (SIDMS) (EPA Method 6800) metrology has been successfully applied as a diagnostic tool with the modified accompanying extraction version of EPA Method 3060A. In our study, aggregate materials distributed over a large area of a major western US state were found to contain a high concentration of total chromium (195 +/- 13 to 709 +/- 19 microg g(-1)) and significant amounts of Cr6+ (141 +/- 6 to 341 +/- 29 microg g(-1)) which are at least three orders of magnitude higher than the US EPA threshold limit (0.5 microg g(-1)). Sediment samples from a major western US state, studied independently, were found to contain less (1.77 +/- 0.34 microg g(-1)) or no Cr6+ in the presence of significant total chromium.