Bile acids combined with fecal sterols: a multiple biomarker approach for deciphering fecal pollution using river sediments

Abstract

Purpose

Fecal sterols (FSs) and bile acids (BAs) are biomarkers that can be used to track sewage pollution. These biomarkers offer an alternative to microbiological analyses, which have several weaknesses, including a lack of specificity. FSs are recognized fecal biomarkers, but there are very few studies showing the use of BAs as a fecal tracer, especially in river sediments, to the best of our knowledge. Thus, the aim of this work was to determine the response of BAs as an alternative biomarker of fecal pollution in parallel to FSs in freshwater sediments and use both biomarker groups to decipher the potential fecal pollution sources in a subtropical river.

Materials and methods

Eight BAs and six FSs were extracted from sediments of the Iguassu River, southern Brazil, and analyzed by GC-MS.

Results and discussion

Among the sterols, coprostanol was dominant (20–4488 μg g⁻¹), followed by stigmastanol (8.67–1788 μg g⁻¹) and cholesterol (4.64–881 μg g⁻¹). 3α-Deoxycholic acid (3α-DCA) was the predominant BA (1.63–871 μg g⁻¹), followed by 3α-lithocholic acid (3α-LCA) (1.77–170 μg g⁻¹). The trends of the total BA and FS contents were different. Very few correlations between the individual BAs and FSs were found, probably due to their different environmental transformation processes and origins, whether animal or human. This watershed encompasses a mixture of land uses, including agricultural applications involving manure and composted sewage sludge as fertilizers and urban occupation, including several treated and untreated sewage discharges. When FS ratios were applied, most sites tested positive for human fecal pollution. Different ratios using secondary and primary BAs were assessed, e.g., (3β-LCA + 3α-LCA)/(3β-DCA + 3α-DCA), 3α-LCA/3α-DCA, and 3α-DCA/CDA. Most ratios detected the influences of different land uses and inputs along the river, including treated and untreated sewage. Furthermore, at the site considered to have moderate fecal pollution, the ratios were in contrast to those obtained from the sites with heavier fecal pollution.

Conclusions

Bile acids and fecal sterols were successfully applied to decipher different origins of fecal pollution using sediments. The contents of individual BAs and derived ratios detected the different land uses and inputs along the river, distinguishing regions with low to high fecal pollution. In general, BAs and FSs did not appear to be strongly related, individually, or as a ratio. However, each compound class provides different information, and using both provides a clearer picture of the potential sources of fecal pollution, especially for a mixed land use region.