Collaborative Research: Evaluating how the sampling integration scale affects field estimates of groundwater transit time and nitrogen fluxes

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The transit time of groundwater, from recharge at the water table to discharge at streams, is a fundamental aquifer characteristic relevant to rates of contaminant transport in groundwater and other aspects of water resource management. The project goal is a rigorous quantitative comparison of three novel approaches for estimating each of three variables related to transit time and non-point-source nitrate contamination: (a) the mean transit time (MTT) of groundwater in aquifers, (b) the magnitude of water and nitrate flux from aquifers to streams, and (c) the amount of nitrate degradation occurring in groundwater prior to seepage into streams. Each of the three approaches is based on groundwater and stream hydrologic and tracer measurements, however, the data collection methods and analysis strategies differ greatly. A side-by-side evaluation of the approaches elucidating strengths and limitations does not exist and will be completed by this project in two streams in an agricultural region of the N.C. Coastal Plain.The project will take place in the Neuse River watershed, site of an ambitious basin-wide regulatory effort by the state of N.C. to reduce nitrogen (N) contamination and associated harmful algal blooms and fish kills in the Neuse River estuary. Several years of effort have not reduced estuarine N input. Groundwater transit times for N, which can extend for decades, are potentially an important contributing factor. By comparing three novel approaches to measuring MTT, nitrate flux, and nitrate degradation, this project will better define the pros and cons of the approaches. The results will provide practical guidance to improve assessment strategies applicable to water resource and land management. The project will provide advanced hydrologic training for students at two universities (N.C. State and the Univ. of Utah), and practical results and insight for state regulators dealing with non-point-source pollutants.