Anthropogenic impacts on urban watersheds : insights from LA-ICP-MS analyses of bald cypress tree cores

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2023-12

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This study provides insights into the impact of anthropogenic factors on urban watersheds using elemental analysis of bald cypress (Taxodium distichum) tree cores as a proxy for watershed water chemistry. It compares five trees from two watersheds in Austin, Texas, three from the highly urbanized Waller Creek watershed (Waller Ck.) and two from the less urbanized Onion Creek watershed (Onion Ck.). Measurements are carried out by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). LA-ICP-MS was calibrated/verified by solution ICP-MS measurements of two certified reference materials. The time series of elemental concentrations revealed (1) distinctly different patterns between sapwood and heartwood, (2) distinct increases in concentrations coinciding with severe drought, (3) similarities between some elemental concentrations in tree rings and streamwater, and (4) similar patterns in concentration biplots between elements in the tree rings and the streamwater for both watersheds. The latter supports the hypothesis that dendrochemistry can serve as a proxy for streamwater chemistry. The elements associated with traffic and municipal water leakage were established from the literature and their historic trends analyzed in conjunction with changes in traffic volumes and NRW losses. In analyzing these trends, the uncertainty in measuring the concentration of each element was considered. Over the 2001-2021 period, several traffic-associated elements (K and Cr) were found to increase with increasing Onion Ck. traffic, while only Zn was found to decrease with decreasing Waller Ck. traffic. Given the uncertainties on measuring Al and Cu, no clear correlation was established between changes of these elements and traffic. The concentrations of municipal water leakage-associated elements Mg and Cu were found to increase in Waller Ck. only suggesting that that this is a problem in the heavily urbanized watershed. Although this analysis was not able to separate the sources contributing to these elemental concentration changes, it provided insights into the effect of urbanization on urban watersheds. Positive matrix factorization (PMF) analysis is used to compare the two watersheds in terms of the relative proportion of elements over time (period 1983-2019). Six factors are fitted and their relationship to elemental concentrations is defined. The results, although dominated by Mg, K and Fe, which are common in topsoil, show that Cu and Fe, which are associated with traffic and municipal water, are higher in Waller Ck. than Onion Ck.

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