The chemical quality of the Susquehanna River at Harrisburg is influenced by three major factors: streamflow, anthracite and bituminous coal-mine drainage, and geology. Water samples collected at Harrisburg near the west bank of the Susquehanna River and those of western tributaries that drain limestone terranes are similar in chemical quality. The water is alkaline and contains calcium, magnesium, and bicarbonate-ion concentrations typical of water drained from limestone. The chemical quality of water samples collected in the center of the river resembles the quality of the West Branch Susquehanna River, which has a dissolved-solids content of about 200 parts per million, and a sulfate-ion concentration that generally exceeds the bicarbonate-ion concentration. Samples collected near the east or Harrisburg bank show the effect of anthracite coalmine drainage from the river's eastern tributaries. The pH of these samples ranges from 5.7 to 7.5, and sulfate is the predominate ion.
The dissolved-solids content of the river at the Harrisburg cross-section stations is inversely proportional to the streamflow. During periods of low riverflow, the dissolved-solids content approaches a maximum; during periods of high flow, the content is low.
The chemical composition of the river at the Harrisburg cross section indicates that water from the principal tributaries above Harrisburg does not mix sufficiently to lose its chemical-quality identity before reaching Harrisburg irrespective of the long distance involved, the many islands and bridge piers, and the rough streambed. This lack of lateral mixing is probably due to the small depth-width ratio and the extreme width of the river.
