Acid rain is less acidic in quarries because there are exposed minerals in the rocks in quarries that neutralize the acid in the rainwater. For example, a limestone quarry is a site where the rock limestone is excavated from the Earth. Limestone is mostly composed of calcite, or the chemical calcium carbonate (CaCO3). Acidic water is water with an excess of protons dissolved in it. The excess protons in the rainwater of acidic rain falling on the exposed CaCO3 will react with the carbonate ion (CO3^-2) to produce carbonic acid (H2CO3). Carbonic acid will decompose to water (H2O) and carbon dioxide (CO2). So the carbonate in limestone will serve to neutralize the protons in the acid rain through an acid/base reaction and leave the water more neutral. This is why acid rain is less acidic in quarries.