How robust is water quality determination with optical measurements for highly polluted agricultural waters?

Optical technology is used to measure and monitor water quality, to date mainly in drinking water treatment, where water is typically much cleaner than in heavily polluted agricultural streams. To characterize suspended sediments (SS) and dissolved organic matter (DOM) in surface water some optical measurements have been used (absorbance and fluorescence spectroscopy). This study aims to use laboratory optical instruments (using turbidimeter and fluorescence and absorbance spectrophotometer) to evaluate how different sediment concentrations measured as turbidity affect fluorescence and absorbance determination of the dissolved organic matter. In the study, ten agricultural catchments located in southern Sweden were analyzed. Clay and sandy soils are most dominant in these catchments with predominant intensive crop production and high livestock in some like F26, E23. Turbidity was measured using a nephelometric turbidimeter to measure the absorption and scatter properties of suspended sediments in the water. Higher turbid waters were found in catchments with clay soil textures (C6, M36, O8, E23, and U8) than sandy soils (E21, F26, I28, M36, and N34). Absorbance and excitation wavelength at 240-600 nm and emission wavelengths at 211-260 nm were used to measure SS and DOC in the water samples. A strong correlation was identified between turbidity and absorbance at 240 and 600 nm range than it was with the fluorescence index vs. turbidity. The result showed a variation in fluorescence index (values range from 1.51 to 1.79) among catchments, these indicate where DOM is coming from. DOM in most catchments is delivered from terrestrial sources, only F26 with a value of FI=1.51 DOM might be derived from a microbial source. The results show a variation among agriculture catchments. UV and FI can be used as a substitute to measure turbidity (SS and DOM) but according to results obtained UV correlates well with turbidity than FI. Implying that UV254 can be a better surrogate parameter to estimate the suspended sediment and DOM in water measured as turbidity for both filtered and unfiltered samples. Additionally, it would be useful to use large quantities of water samples to be able to identify the size effect between turbidity and fluorescence correlations