Exponent

About: Exponent is a company organization based out in Menlo Park, California, United States. It is known for research contribution in the topics: Population & Risk assessment. The organization has 1589 authors who have published 2680 publications receiving 88140 citations.

Within-day drinking water consumption patterns: Results from a drinking water consumption survey

Abstract: Data currently available on drinking water intakes do not support dietary exposure estimates for contaminants that have acute effects lasting less than 24 h. Realistic exposure estimates for these types of contaminants in drinking water require detailed information on amounts and time of consumption for each drinking occasion during a day. A nationwide water consumption survey was conducted to address how often, when, and how much water is consumed at specific times during the day. The survey was conducted in two waves, to represent two seasons, and the survey instrument consisted of 7-day water consumption diaries. Data on total daily amounts consumed, number of drinking occasions per day, amounts consumed per drinking occasion, and intervals between drinking occasions show larger between-subjects variation than within-subject variation. Statistically significant associations were also observed between drinking water consumption patterns and participants' ages and sex and geographical regions in which these participants live. The number of drinking occasions on a given day varied from 0 to 19, with the majority of respondents reporting 6 or less drinking occasions per day. The average interval between drinking occasions varied from 1 to 17 h, with 57% of the person-days reporting average intervals at least 3 h apart. The mean amount consumed per drinking occasion showed little association with the number of drinking occasions and fluctuated between 8 and 10 oz. To our knowledge, this survey is the only source of information on within-day patterns (i.e., when and how much) of drinking water consumption for a nationally representative sample of the US population. The detailed water consumption data from this survey can be used to support less than 24-h dietary exposure estimates for contaminants in drinking water.

Importance of considering the framework principles in risk assessment for metals.

Abstract: The recent EPA Framework for Metals Risk Assessment provides the opportunity for contextual risk assessment for sites impacted by metals (such as the depicted Dauntless Mine in Colorado).

Environmental fate of pyrethroids in urban and suburban stream sediments and the appropriateness of Hyalella azteca model in determining ecological risk

Abstract: According to several recent studies using standard acute Hyalella azteca sediment bioassays, increased pyrethroid use in urban and suburban regions in California has resulted in the accumulation of toxic concentrations of pyrethroids in sediments of area streams and estuaries. However, a critical review of the literature indicates that this is likely an overestimation of environmental risk. Hyalella azteca is consistently the most susceptible organism to both aqueous and sediment-associated pyrethroid exposures when compared to a suite of other aquatic taxa. In some cases, H. azteca LC50 values are less than the community HC10 values, suggesting that the amphipod is an overly conservative model for community- or ecosystem-level impacts of sediment-associated pyrethroids. Further, as a model for responses of field populations of H. azteca, the laboratory bioassays considerably overestimate exposure, because the amphipod is more appropriately characterized as an epibenthic organism, not a true sediment dweller; H. azteca preferentially inhabit aquatic macrophytes, periphyton mats, and leaf litter, which drastically reduces their exposure to contaminated sediments. Sediment-bound pyrethroids are transported via downstream washing of fine particulates resulting in longer range transport but also more efficient sequestration of the chemical. In addition, site-specific variables such as sediment organic carbon content, grain size, temperature, and microbial activity alter pyrethroid bioavailability, degradation, and toxicity on a microhabitat scale. The type and source of the carbon in particular, influences the pyrethroid sequestering ability of sediments. The resulting irregular distribution of pyrethroids in stream sediments suggests that sufficient nonimpacted habitat may exist as refugia for resident sediment-dwelling organisms for rapid recolonization to occur. Given these factors, we argue that the amphipod model provides, at best, a screening level assessment of pyrethroid impacts and can correctly identify those sediments not toxic to benthic organisms but cannot accurately predict where sediments will be toxic.