CONTRIBUTIONS OF GROUNDWATER TO THE SURFACE WATER SYSTEM IN THE UPPER ULUA RIVER BASIN OF CENTRAL HONDURAS

The purpose of this project was to conduct a hydrogeologic assessment in and around La Union, Lempira Department, Honduras to build the knowledge base of information for the contribution of groundwater to the surface water system. To address this problem, we employed field methods focused on spring characterization for geologic setting and geochemical signatures. Spring locations provide clues to the subsurface lithology while geochemistry can indicate groundwater residence. The stratigraphy and structure influences the spring locations and geochemical signatures. Streams and springs were monitored for pH, flow characteristics and conductance as a measure of total-dissolved-solids. Structural information was collected on rock unit outcrops using a Brunton transit compass for strike and dip of lithologic contacts between stratigraphic units. Field data was gathered and locations determined using cellular signal and the Fulcrum mapping software mobile app for latitude and longitude. During the summer of 2017, 130 geologic points and 75 water points were gathered in the field. Structural trends include a cluster of strikes around 240 degrees, dip direction and approximate magnitudes at 330 and 40 degrees respectively. This geologic data indicates significant tilting and structural faults, supportive of tectonic activity and mountain-building in the region. It also provides evidence of stratigraphic recurrence for the Valle de Angeles and the Jaitique formations associated with structural deformation in the area. Water point data ranged from 22.6 to 485 mg/L with higher conductivity values indicating groundwater influx into the surface water system. In comparison, water points with lower conductivity and pH are attributed to runoff from rainfall events. Springs occur on the western side of the mountain where the Jaitique unit crops out and exposes its contact with the Lower Valle de Angeles. The Lower Valle de Angeles acts as an aquiclude preventing percolation into the ground and resulting in a spring at the surface. For many of the springs in this area, we interpret that water is stored in the Jaitique formation and moves down dip, to the west, through secondary porosity.