Lithofacies and Transport for Clastic Sediments in Karst Conduits

Abstract

Karstic aquifers carry a load of clastic sediment as part of their hydrologic function and these are an important part of the mechanism for storage and transport of contaminants. Indeed, solid contaminants can be considered as a form of clastic sediment. Although the sources of clastic sediments are usually well delineated, sediments from multiple sources are mixed and redistributed within the aquifer to produce the sediment deposits that remain stored in caves or the load of sediment discharged from karst springs. As an aid to the interpretation of clastic sediments in karst aquifers, we have modified a previously proposed facies concept with an emphasis on its implications for contaminant transport and storage. Five facies are defined in terms of particle size, degree of sorting, and sedimentary structures: backswamp facies, channel facies, diamicton facies, slackwater facies, and thalweg facies. The deposits represented by each set of facies characteristics in turn can be interpreted in terms of depositional mechanisms. The slackwater facies and channel facies are the most significant in terms of implications for contaminant transport and therefore receive greater emphasis than the other three in this discussion. The facies labeled slackwater facies are laminated deposits of clay to silt laid down in passages filled with stagnant water either flooded by inputs from upstream or backflooded from surface streams. This mechanism provides two pathways by which microorganisms or metals can be adsorbed onto clay particles and carried into the aquifer. The facies labeled channel facies consist of silts, sands, gravels, and cobbles carried in major conduits mostly by high velocity storm flows. Flows that transport sediments resulting in channel facies also can carry solid contaminants at various size scales and can act as storage sites for contaminants over long periods of time. Calculations show that hydraulic conditions required for transport leading to deposition of channel facies are consistent with observed discharge characteristics of major conduits.