WATER RESOURCES OF ORANGE COUNTY 133
migrates to supply wells causing them to lose prime or to cease production.
OTHER ASPECTS OF DRAINAGE WELLS
An aspect of drainage wells that must be considered in any longrange appraisal is accelerated solution of the limestone that forms the aquifer. Solution of limestone is largely controlled by the amount of carbon dioxide dissolved in the water. When water filters slowly through the semipermeable beds overlying the limestone, some carbon dioxide escapes and some reacts with shell contained in the sandy clay. Therefore, the strength of the carbonic acid solution entering the limestone is reduced. Also the recharge is distributed fairly uniformly over a wide area. Drainage wells, however, deliver large quantities of water with a relatively high carbon dioxide content to a small area. Therefore, solution of the limestone is accelerated in the vicinity of drainage wells.
Solution of limestone is a process which is going on all the time and collapse of the surface material (sinkhole formation) is a natural phenomenon. Under natural conditions the chances of a sinkhole forming under a building are very small considering the very small percentage of the county which is covered by buildings and the small number of new sinkholes formed each year. However, because drainage wells are concentrated in urban areas and speed up the process of solution, they increase the possibility of buildings being damaged by sinkhole formations.
Sinkholes are generally caused by solution and collapse of limestone in the upper part of the aquifer; therefore, drainage wells which are cased several hundred feet into the limestone would probably not appreciably increase the danger of sinkhole formation. However, most drainage wells in Orange County are cased only to the top of the limestone.
Old drainage wells and old abandoned wells in general are a danger in another way. The water table is above the artesian pressure surface in most of the county and if corrosion creates holes in the casings below the water table, nonartesian water can run into the well. This running water may carry surface sand down the well and out into solution channels in the limestone and eventually cause surface subsidence or collapse even though there is no collapse of the limestone. The danger of such subsidence will increase with time; therefore, wells should be inspected periodically to be certain their casings are sound. Abandoned wells