دانلود رایگان مقاله تاثیر تغییرات شیمیایی انسانی و زمین شناسی آبخوان بر غلظت فسفر زیرزمینی

abstract

Geologic and geochemical variations across a 4200 km2 area of south-central Wisconsin (USA) were used to examine their relationship to phosphorus concentrations in groundwater from more than four hundred private water supply wells. Surficial geology in the study area ranged from Cambrian sandstones to Ordovician dolomites. Groundwater phosphorus concentrations were higher in aquifers of older Cambrian age compared to the concentrations in aquifers of younger Cambrian and Ordovician age. Because iron concentrations were relatively low in these waters and agricultural land use was similar in all geologic regions, we propose that the differences in bedrock phosphorus and anthropogenic geochemical impacts explain the differences in phosphorus concentrations between aquifers. Within the older Cambrian aquifers, groundwater phosphorus concentrations were elevated in groundwater with higher nitrate-nitrogen concentrations. This finding is consistent with the presence of phosphorus within sediment in these strata and geologic conditions that weakly buffered pH reduction from anthropogenic acidification. In contrast, groundwater phosphorus concentrations in younger Cambrian and Ordovician aquifers were not elevated in samples with higher nitrate. Anthropogenic acidification in these carbonate-rich aquifers was neutralized through increased carbonate weathering, which led to higher groundwater calcium and alkalinity and would limit the dissolution of phosphate-rich minerals, such as apatite, where present. Low iron concentrations observed in most samples suggest that the phosphorus release in the Cambrian strata occurs beyond the zone of secondary mineral retention in the soil. These results have important implications for the eutrophication of inland surface waters in areas with bedrock phosphorus and anthropogenic acidity that is not neutralized before it contacts phosphatic rock.

4. Conclusions and implications

The results of this study demonstrate that geologic setting and anthropogenic change to groundwater geochemistry combine to influence groundwater phosphorus concentrations. Although groundwater throughout the geologic regions had some elevated nitrate concentrations, only groundwater from the lower Cambrian showed an association between higher phosphorus and elevated nitrate. In the more carbonate-rich upper Cambrian/Ordovician groundwater, increased nitrate concentrations were accompanied by increases in calcium, magnesium and alkalinity, as expected from accelerated weathering of carbonate minerals, but there was no associated increase in the groundwater phosphorus concentrations. In contrast, groundwater sampled from the lower Cambrian bedrock displayed a weaker relationship between nitrate and calcium, magnesium and alkalinity but a marked increase in groundwater phosphorus with elevated nitrate. These results are consistent with anthropogenic acidity leading to phosphorus mineral weathering beyond zones where secondary minerals would retain phosphorus. This suggests that differences in groundwater phosphorus concentrations at the watershed-scale reflect contrasts in the spatial distribution of bedrock phosphorus and rock weathering that control major ion chemistry. These results have important implications for the transfer of terrestrial phosphorus to surface waters. In our study area, we found groundwater phosphorus concentrations could be increased by a factor of five at a nitrate concentration of 10 mg N L1 . Because phosphorus-containing mineral phases have relatively wide occurrence within Cambrian bedrock, groundwater phosphorus may be an increasingly important component of phosphorus transfer from terrestrial to aquatic systems as anthropogenic geochemical change impacts a larger proportion of groundwater aquifers. Increased transport of phosphorus at a watershed scale would pose an important challenge to surface water management at decadal time scales as these changes propagate through aquifers, and downstream water resource systems are affected by changes in groundwater chemistry.