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Groundwater discharge zones are important spawning areas for fish because they provide a thermally stable habitat. Research at three streams in the Chesapeake Bay Watershed with areas of focused groundwater discharge revealed that groundwater also is a source of phytoestrogens and herbicides that could result in fish exposure during sensitive life stages.
In the Chesapeake Bay Watershed, smallmouth bass (Micropterus dolomieu) populations have shown signs of infectious disease and endocrine disruption. Exposure to contaminants during sensitive life stages can have a negative effect on fish health; therefore, understanding groundwater discharge as a contaminant exposure pathway for nesting smallmouth bass is of great interest to stakeholders in the Chesapeake Bay Watershed. However, little work has focused on understanding the role of groundwater discharge zones as source of contaminants to streams.
To fill this gap, scientists used recently refined heat tracing methods to identify and quantify groundwater discharge zones in three streams in the Chesapeake Bay Watershed near smallmouth bass spawning areas. Paired groundwater and water samples from these three streams were analyzed for phytoestrogens (natural estrogens that occur in plants), herbicides (such as atrazine) and their breakdown products (such as hydroxyatrazine), steroid hormones, sterols (such as cholesterol) and bisphenol A. Chemicals detected in groundwater discharge zones were compared to those in the paired stream samples to better characterize the contribution of groundwater discharge as a contaminant source in the studied streams.
An important finding of this research was that phytoestrogens and herbicides were present in the streams and the corresponding groundwater discharge zones, with concentrations generally lower in the groundwater samples. The most frequently detected chemicals in this study were cholesterol (88 percent, fecal sterol), genistein (79 percent, phytoestrogen), atrazine (74 percent, herbicide), formononetin (55 percent, phytoestrogen), and metolachlor (50 percent, herbicide).
Lower contaminant concentrations in groundwater discharge compared to those in adjacent streams indicate that runoff events are likely the largest episodic contributors of contaminant concentrations at the streams sampled, but groundwater discharge zones appear to be a consistent contributor of contaminants such as pesticides and phytoestrogens. Thus, these groundwater discharge zones represent a focused chemical exposure pathway in a habitat used by aquatic organisms during sensitive life stages, such as spawning. Although not quantified in this study, exposure of organisms to contaminants by way of groundwater discharge during sensitive early life stages could have harmful sublethal effects on growth, development, and reproduction with time.
This research was supported by the U.S. Geological Survey's Chesapeake Bay Priority Ecosystems and Fisheries programs, and the U.S. Geological Survey Ecosystems Mission Area’s Environmental Health Program (Contaminants Biology and the Toxic Substances Hydrology).
See below for other science related to this research.
Thompson, T.J., Briggs, M.A., Phillips, P.J., Blazer, V.S., Smalling, K.L., Kolpin, D.W., Wager, T. 2021. Groundwater discharges as a source of phytoestrogens and other agriculturally derived contaminants to streams. Science of the Total Environment, v. 755.