Characterisation of suspended particles and associated metals in groundwater under natural gradient flow conditions

Microscale Eulerian variations in the flux, mineralogical composition and size of suspended particles have been found in a contaminated sandy aquifer under natural gradient flow conditions (q̄=11 to 16 m / y) during an 8 month study period. Particle variability has been detected along a 16 m saturated section of the aquifer at a scale of centimeters and meters in the vertical and horizontal dimensions, respectively. The average concentration of particles in groundwater varied between 1 to 70 mg/l. The particles were primarily composed of CaCO₃ (11% to 57%), quartz (7% to 39%) and clays (8% to 43%). Most of the particles were within the 140 to 3,000 nm size range with size modes varying from 310 to 660 nm. The large amounts of suspended particles are considered to be related to high inputs of dissolved organic carbon into groundwater from sewage effluents which have been used for agricultural irrigation since the early 1960's. As a result of organic matter biodegradation in the saturated zone,anoxic conditions developed and the PCO₂ content of groundwater increased dramatically. It is postulated that part of the carbonate cement of the rocks dissolved and detrital CaCO₃, quartz and clay were released as colloidal particles. In the prevailing anoxic conditions of groundwater at the study site (DO < 1 mg/l) colloidal stability is enhanced by organic matter coating of particles. The transport of metals associated with suspended particles in the saturated zone and the interaction of these particles in the aquifer environment have been ascertained through a comparison of the distribution coefficient of 17 elements as a function of depth.