Water quality impacts of irrigation return flow on stream and groundwater in an intensive agricultural watershed

Highlights: • Chemistry in stream water near rice paddy field is not entirely explained by THSMs. • Plot of Cl/NO₃ and NO₃/HCO₃ indicates natural and anthropogenic sources. • δ¹⁵N_NO3 and δ¹⁸O_NO3 values indicate that manure is dominant source in many samples. • Water chemistry is deciphered by combination of THSMs, multi-isotopes and elements. Irrigation return flow can include contaminants derived from agricultural practices, and then deteriorate the quality of surface and subsurface water within the watershed. Thus, it is important to estimate the effect of irrigation return flow on water chemistry/quality. To do that water samples were collected between November 2004 and December 2005 from stream and groundwater in a small watershed that contains extensive rice paddy fields. The water isotopic compositions represented seasonal variation, particularly in downstream of main channel and the tributary. In April and May, water samples in the downstream and tributary could not be explained by three-component (soil water, groundwater and rainfall) hydrograph separation models (THSM). These results indicated that the stream water was affected by high evaporation and that another water body (e.g. quick return flow) impacted on THSM. Plot of Cl/NO₃ and NO₃/HCO₃ showed that the water chemistry of all water samples was mainly regulated by soil water and groundwater. In addition, the water chemistry was related to water derived from rice paddy fields (WR) and manure. Manure impacted the water chemistry in tributary, one of the shallow groundwaters and the deep groundwaters, whereas that water in downstream was affected by WR. On a plot of δ¹⁵N_NO3 and δ¹⁸O_NO3 values, many samples were in a cluster indicative of manure and on a denitrification line. These imply that irrigation return flow characterized by denitrification processes was involved in determining the water chemistry. We suggest that chemical and multi-isotopes approach combined with the THSM is useful to elucidate the sources and processes controlling water chemistry in stream associated with rice paddy fields.