Karst hydrogeology, hydrogeochemistry and processes of tufa deposition in carboniferous limestone springs of the Mells Valley, Somerset
A number of karst springs rise in the Carboniferous strata of the Mells Valley,\ud Somerset which is an area surrounded by many active and disused quarries.\ud Some of these karst springs actively deposit tufa, a secondary precipitate of\ud calcium carbonate, and they have been identified by the Environment Agency\ud as potentially vulnerable to the effects of local sub-water table quarrying. A\ud comparative study of a tufa-depositing spring, Whitehole Farm Spring, and a\ud non tufa-depositing spring, St. Dunstan's Well, was carried out in order to\ud determine the influence of hydrogeology, hydrogeochemistry and\ud environmental biology on the processes of tufa deposition. The two springs\ud are approximately 2 km apart and located within the same outcrop of\ud Carboniferous Limestone strata.\ud A series of qualitative dye-tracing tests established a positive hydraulic\ud connection between a feeder stream which rose in the Old Red Sandstone of\ud the upper hydrological catchment, a sinkhole at the junction with the Lower\ud Limestone Shales and Whitehole Farm Spring resurgence in the\ud Carboniferous Limestone. The sinkhole was shown to have two separate\ud channels to the water table. The tests demonstrated that structural geology\ud and the water table had definable effects on the subterranean route and travel\ud time of the allogenic recharge water. The recharge water to Whitehole Farm\ud Spring was guided by the SE - NW Downhead Fault and did not follow the\ud SW - NE course of the natural dry valley. The water velocity was dependent\ud on the height of the local water table at the time of the tests. Hydrograph\ud analysis of flow data combined with the dye-tracing tests illustrated the rapid\ud response of the springs to storm events (< 24 hrs). The results gave an\ud indication of the shallow nature of the Carboniferous Limestone aquifer and\ud the different stages of development of fissures and conduits in the two spring\ud systems, St. Dunstan's Well being a more mature karst system than\ud Whitehole Farm Spring.\ud Analysis of 18 monthly water samples from both springs at their resurgences\ud and 100 metres downstream revealed temporal and spatial differences in the\ud hydrogeochemistry of the springs and their streams. Ca 2+ and HCOs' were\ud the dominant ions in both spring waters, however, neither of the spring\ud resurgences were supersaturated with respect to calcite (Whitehole Farm\ud Spring, mean -0.17 ± 0.08 Sic ; St. Dunstan's Well, mean -0.33 ± 0.08 Sic).\ud The resurgence water at Whitehole Farm Spring was higher in Ca 2+ (mean\ud 112 ± 3.35 mg I" 1 ) than at St. Dunstan's Well and became supersaturated as it\ud flowed downstream (mean 0.43 ±0.10 Sic). Lower Ca 2+ levels at St.\ud Dunstan's Well (80 ± 2.7 mg I" 1 ) were influenced by the degassing of recharge\ud water and the deposition of calcite as speleothem within the open system of\ud caves behind the resurgence. Downstream supersaturation was also lower\ud (-0.011 ± 0.079 Sic). There was a highly significant difference (p <0.001)\ud between mean daily flow at Whitehole Farm Spring (0.95 ±0.19 ML d" 1 ) and\ud St. Dunstan's Well (11. 58 ± 1.93 ML d" 1 ). Electrical conductivity, total\ud dissolved solids, alkalinity, Na, K, and NO 3 were also higher at Whitehole\ud Farm Spring, the differences being significant (p < 0.01).\ud There were major differences in the biodiversity and environment of the two\ud sites. Petrographic examination of field-collected tufa samples from\ud Whitehole Farm Spring demonstrated that the stream flora, in particular lower\ud plants, and the surrounding riparian vegetation were a major influence on the\ud formation and morphology of the tufa deposits. Newly accreted tufa which\ud had formed on artificial substrates placed in the stream, revealed calcite\ud crystals surrounding the empty moulds of filamentous cyanobacteria.\ud Hydrological conditions within the stream also influenced the\ud micromorphology of calcite crystals forming on two filamentous algal species,\ud Vaucheria longata and Zygnema stellinum.\ud Whitehole Farm Spring was also found to be the more stable environment\ud where biodiversity was higher. The shaded conditions created by Whitehole\ud Farm Spring's woodland environment were beneficial to the growth of\ud filamentous algae, cyanobacteria and bryophytes. The dominant species\ud upstream was a crustose epilithic red alga Hildenbrandia rivularis.\ud Downstream, in and on the tufa deposits the dominant aquatic flora were a\ud number of species of the filamentous yellow-green alga Vaucheria and the\ud cyanobacterium Lyngbya and the moss species Palustriella (Cratoneuron)\ud commutatum. These species were either absent or growing under different\ud ecological conditions at St. Dunstan's Well where there was sparse woodland.\ud The dominant species at St. Dunstan's Well was the filamentous green alga\ud Cladophora glomerata which was not present at Whitehole Farm Spring.\ud Debris from the surrounding riparian vegetation at Whitehole Farm Spring\ud acted as substrates for colonisation by microorganisms which enhanced the\ud nucleation and growth of tufa in the stream. Hydrological and environmental\ud conditions at St. Dunstan's Well prevented the accumulation of organic debris\ud within the stream flow. The study highlighted the need for conservation of the\ud natural environment and biota in order to maintain actively-depositing tufa\ud springs.
views in local repository
downloads in local repository
The information is available from the following content providers: