New Mexico Geological Society Annual Spring Meeting — Abstracts

Forest fires are well known to impact the quality of water in a watershed from hillside erosion and fire sedimentation as well as the quantity of water entering the system from decreased soil infiltration, lessened vegetation interception, and increased catchment evapotranspiration. The 2022 Hermits Peak/Calf Canyon (HP/CC) Fire, the largest wildfire in New Mexico’s history (>340,000 acres), impacted the Headwaters Gallinas Watershed. The Gallinas River is the source water for the City of Las Vegas (population ~13,000) drinking water supply, as well as a designated water resource for high quality coldwater aquatic life, irrigation, livestock watering, and wildlife habitat (NMAC 20.6.4.215). Approximately 115,542 acres burned in the Gallinas River headwaters with 21% classified as high burn severity. This study assessed the HP/CC bearing on water quality using data from USGS real time surface water gaging station 08380500 (Gallinas Creek at Montezuma). Amounts and trends in daily average values for temperature, specific conductance, dissolved oxygen, pH, and turbidity were assessed relative to discharge over two years post-HP/CC fire (September 01, 2022-August 31, 2024) to understand its impact on potable supplies and ecosystem services. Water temperature reached a high of 21.7°C during summer months but remained below the 26°C high quality coldwater threshold. Specific conductance had a high of 309 µS/cm @ 25°C, low of 73 and an average of 218.82. The values exceeded the 300 µS/cm threshold 3 out of the 361 days of record (~1.0%). Dissolved oxygen had a high of 11.6 mg/L, low of 7.2 and an average of 8.85. The values remained above the 6.0 mg/L threshold throughout the period of record. Values of pH fluctuated between 8.6-7.9 (average 8.28) and remained above the 6.6 pH threshold throughout the period of record. Turbidity values ranged from a low of 0.8 NTU to a high of 934.0. More than 30% of the 485 days of record exceeded 20 NTU, the City of Las Vegas’ current water treatment plant limit and more than 50% of the 485 days of record exceeded the high quality coldwater tolerance of 10 NTU (NMAC 20.6.4.900). Turbidity in the City of Las Vegas water system has been a significant water treatment issue post-HP/CC wildfire, leading to mandatory boil water advisories and water supply challenges. Monitoring water quality remains imperative to manage fire sedimentation, protect water supplies, and inform adaptation strategies.