A Study of Seasonal Salinity Variation in Lake Whitney, Texas Using Continuous Resistivity Profiling

Abstract

Lake Whitney, Texas is an example of many reservoirs whose water quality is controlled by highly variable sodium chloride loads originating from near surface evaporate deposits within the upper reaches of the contributing watershed. We investigate seasonal changes in salinity within Lake Whitney using the continuous resistivity profiling (CRP) method. From December, 2006 to October, 2007 we repeated a 30‐km long profile along the axis of the reservoir six times. A 135‐m long, 11‐electrode, marine resistivity array was used to collect eight dipole‐dipole readings at offsets from 5–120 m, at intervals of approximately 1 m along the profile. Each profile was collected in an 8 to 10 hour period to produce a “snapshot” of the salinity distribution. Starting from a well mixed and highly saline (3 ohm‐m/2,300 mg/L) state in early winter 2006, the reservoir became progressively fresher, due to the inflow of fresher water (4 ohm‐m /1,700 mg/L) over the winter months. Spring rains, caused resistivity to rise and salinity to drop as layers of freshwater flowed out along the surface from north to south after each storm. The freshwater in these layers progressively mixed with deeper, more saline water as it moved south, raising the average resistivity of the reservoir. A second layer of cold freshwater flowed along the reservoir bottom in the north and out into mid‐water depths as it moved south. The greatest contrast within the water column always occurred near the main tributary inlet and was least near the dam, suggesting progressive downstream mixing. We conclude that the CPR method can be used to delineate the depth and lateral extent of abnormally fresh and saline regions within water reservoirs and can resolve shallow and deep zones of contrasting salinity within the water column.