Speaker: Gabrielle Dunn, MS student & CU Boulder Water Fellows Program Associate, Department of Civil, Environmental and Architectural Engineering, University of Colorado, Boulder

Topic: St. Vrain Hydraulic Connectivity Study

Abstract
The City of Longmont tasked the University of Colorado Boulder with evaluating the potential impacts of wastewater treatment plant (WWTP) effluent on domestic groundwater wells along the St. Vrain Creek segment COSPSV03. The study had three primary objectives:

1. to assess the current usage and status of domestic groundwater wells summarized by the Colorado Department of Public Health and Environment (CDPHE),

2. to evaluate the risk of exceeding the drinking water standard for nitrate plus nitrite concentrations (10 mg/L) downstream of the WWTP discharge, and

3. to investigate the hydraulic connectivity between the wells and St. Vrain Creek. Data from the City of Longmont’s WWTP effluent, background river conditions, and domestic well permit information were analyzed to assess potential risks.

The study utilized a combination of methods, including an in-depth review of 112 domestic well permits, phone and mail surveys to determine current well usage, and GIS mapping to assess well proximity to the WWTP effluent discharge point. Additionally, the USGS OTIS model was employed to simulate nitrate transport within the creek under varying flow and effluent conditions. The results indicated that the probability of exceeding the 10 mg/L nitrate standard directly downstream of the WWTP effluent confluence is less than 1.3%. Further analysis showed that if the WWTP effluent concentration remains below 13.8 mg/L, the standard would not be exceeded in the segment between the WWTP and Boulder Creek.

Regarding hydraulic connectivity, only four of the 112 wells were identified as potentially at risk from WWTP effluent. Two wells were deemed unlikely to be impacted due to their distance from the creek, while two others may be impacted because of the travel times for water from the river to reach the wells. This study provides valuable data for potential water supply designation changes and further decision-making regarding the re-segmentation of stream segment COSPSV03.

Bio
Gabrielle is currently pursuing her master’s degree in civil engineering at the University of Colorado Boulder, specializing in hydrology, water resources and environmental fluid mechanics. She graduated summa cum laude in May 2024 with a Bachelor of Science in civil engineering from the same university. With prior experience in land development and civil engineering, Gabrielle is now engaged in a research project investigating the hydraulic connectivity of St. Vrain Creek and nearby groundwater wells. Her current interests include groundwater engineering, water resources engineering and stormwater management.