Laboratory dissolution of basalt in a simulated weathering system

Abstract

A laboratory weathering system was developed to simulate weathering of basalt rock on a watershed in north central Arizona. Powdered basalt rock was weathered using solutions with pH of 4.5, 5.5, and 7.0 under two conditions: continuous and alternate wetting and drying. Effects of solutions and weathering conditions on rate of weathering were analyzed. Results indicated that weathering rates for Mg, Na, and K (mobile ions) declined over time and approached a relatively stable rate that was not greatly affected by pH differences. Weathering with alternate wetting and drying showed more cumulative ion removal than continuous weathering for Mg, Na, K, Ca, Fe, and Si. Only Al showed no difference in cumulative weathering due to schedule. The steady-state weathering rate was higher with alternate wetting and drying than continuous weathering. Thus, weathering with intermittent drying resulted in more cumulative ion removal over time than continuous weathering with equal volumes of solution. Intermittent drying appeared to have a greater effect than changes in pH on the steady-state weathering rate. Many of these results are consistent with a weathering mechanism in which a residual layer rich in immobile elements forms on fresh mineral surfaces. The artificial weathering systems were a reasonable simulation of actual field weathering.