ESTIMATING AQUIFER PROPERTIES IN A COASTAL KARST BASIN FROM OBSERVATIONS AT THE WATER TABLE: QUINTANA ROO, MEXICO

Epiphreatic caves provide access to the water table where measurements of tidal amplitude and attenuation allow the calculation of aquifer properties needed to estimate a water budget, where boundary conditions are constrained. Quintana Roo, Mexico contains extensive epiphreatic caves extending at least 10 km inland perpendicular to the Caribbean shoreline. Water budgets are challenging to calculate due to wide variability of precipitation (1092- 1500 mm/yr) and ET (350-2,500 mm/yr), and uncertain boundaries of coastal karst basins. Previous studies have assumed regional gradients are near zero and relatively stable over time. Discharge in low gradient systems is sensitive to even slight changes resulting from recharge and tides, therefore it is important to precisely measure spatial and temporal gradient variability. This study analyzed high-frequency, long-term, water level data from epiphreatic conduits to constrain bulk aquifer properties.

Data were collected at 30-minute resolution in August 2015 and from January 2016 to August 2018 using paired barometric and water-level loggers in Sistema Jaguar near Paamul, Mexico, at sites along a transect (0.5 km, 2 km, 4 km, and 6 km) perpendicular to the coast. Elevations were established using averaged barometric differences relative to a reference station near the coastline. Sites were also surveyed to high-precision GPS benchmarks set at cave entrances. In August 2017, instruments were placed in Pixan Bel, a parallel system to the west of Sistema Jaguar, at 5 to 7 km inland. Hydraulic gradient is steepest near the coastline (7 m/km at 0.5 km inland) and flattens inland (0.3 to 0.5 m/km beyond 5 km inland). Diffusivity ranged from 1 to 2 x 10⁸ m²/d, Transmissivity ranged from 1.5 to 5x10⁷ m²/d (assuming a storativity of 0.3), and Hydraulic Conductivity ranged from 6.1 x 10⁵ to 1.8 x 10⁶ m/d across aquifer thicknesses of 10-30 m. Results are comparable to Beddows, 2004 (D= 1.8 to 3.7 x 10⁷ m²/d, T= 1.1 to 5.4 x 10⁷ m²/d, K= 5.3 x 10⁶ to 2.8 x 10⁷ m/d) and models by Gondwe, 2010 (K= 2.2 x 10⁵ to 5.9 x 10⁶ m/d). Our findings are in general agreement with previous work based on observations in conduits but indicate that diffusivity is high between cave conduits and the coastline, suggesting that matrix flow also contributes substantially to discharge and should be included in aquifer models.