Fingerprinting of Phosphorus in river sediments using the structural equation modeling

Abstract

Abstract Phosphorus (P) is transported into the water resources mainly due to soil erosion. Accumulation of P in water bodies leads to the subsequent eutrophication phenomenon. Using the structural equation modeling, this study investigated the relative contribution of different P-producing sources in the Pasikhan river watershed, south of the Caspian Sea, Northern Iran. For this purpose, 79 surface soil samples and 14 suspended sediments were taken from the potential P sources and the river. These sources included undisturbed and degraded rangelands, forests, rice fields, tea gardens, and gullies. Phosphorus transfer is mainly associated with the movement of clay particles. Therefore, clay was used as an intermediary parameter to make the modeling more reliable. The implemented structural equations in PLS software were used for modeling purposes. The overall model fitting index (goodness of fit,GOF=0.591) showed the model's strong forecasting capability. The results of the T-values test also showed that undisturbed rangelands (T-values=1.67) and forests (T-values=1.31) have no significant effect on the river's P content. Degraded rangelands, gullies, rice fields, and tea gardens significantly contribute to P in the river sediments. In particular, the highest contribution was related to degraded rangelands (intensity of the effect=0.63) and gullies (intensity of the effect=0.47). Finally, the results showed that gullies' contribution was 28.26% to the P production in river sediments, while the other sources had a relatively equal contribution (degraded rangelands=27.5%; tea gardens=23.9%; rice fields=20.3%). Overall, the results confirmed that structural equation modeling is a robust and efficient approach to identifying P sources.