Due to recent changes in agricultural practices the nutrient load in our waterways has increased causing eutrophication and hypoxia. Many legislative actions have taken place because of this problem, including the Clean Water Act of 1972 (CWA), and many different nutrient reduction plans. The CWA governs that impaired waterways must be monitored to meet total maximum daily loads (TMDL) for each watershed. TMDLu27s must be assessed using data collected over a period of time so that reduction techniques can be administered. TMDL assessments are usually conducted by the United States Geological Survey (USGS) through many different monitoring programs. The USGS programs include collecting streamflow and nutrient concentration data and using the information to estimate nutrient loads. Generally, grab sampling is the method of choice for concentrations. Grab samples do not accurately assess the total load as generally only 6-8 samples can be collected over a year due to financial and logistical constraints, while concentrations vary within a span of hours and days. Research applications involve the use of automated sensors (e.g., ISCO) that allow for more frequent sampling in order to overcome this issue but are expensive to purchase and maintain. Thus the development of an inexpensive, passive sampler would be of much interest in estimating load. The Passive Surface Flux Meter (PSFM), an integrative sampler that estimates the total solute load over a storm event, is such an alternative. The PSFM is composed of two sorbents one to collect the contaminant of choice and another to determine the flow through the device. Ion-exchange resin was used to collect nitrates, while Granular Activated Carbon dosed with a suite of alcohols were used to determine flow through the sampler. This thesis sets forth the fundamental theories behind the PSFM, and investigates its ability to measure nutrient fluxes in the field. In-situ deployments within Clear Creek watershed in Iowa were conducted using a modification of the PSFM design by Boland (2011). There was a strong linear relationship between the loads estimated by the PSFM, and u22trueu22 load based on USGS stream gage data, and Nitratax sensor data. This implies that the device could be calibrated to work in the field. However, it was determined that the design underestimated the true load in the stream by 29%. This was attributed to the nonlinear relationship between the external velocity and the flow through the sampler, which weighted the results towards the high flow events. To overcome this constraint, a new design is proposed in which flow through the sampler varies linearly with the transient head at the inlet. Flume experiments done under different flow depths proved that linearity conditions were satisfied. Using the results from the laboratory experiments recommendations were made for design of an in-situ deployment of the new design.
展开▼
