Impact of water-particle interactions on molybdenum budget in humid tropical rivers and estuaries: insights from Nethravati, Gurupur and Mandovi river systems

The study presents the seasonal and inter-annual monitoring of molybdenum (Mo) distribution and variability in humid tropical riverine and estuarine systems (Nethravati, Gurupur and Mandovi estuaries) of west coast of India. The study investigates the geochemical behaviour of Mo in the river and estuaries, and their ultimate fluxes into the ocean. The riverine flux of dissolved Mo (DMo) to the Nethravati, Gurupur and Mandovi estuaries are 1800 mol yr− 1 (4.88 mol day− 1), 195 mol yr− 1 (0.53 mol day− 1) and 10.5 × 103 mol yr− 1 (28 mol day− 1) respectively, and the riverine particulate Mo (PMo) flux to Nethravati estuary is 10.8 × 103 mol yr− 1. The DMo in river (~30 to 40%) is scavenged onto particles under oxidized acidic river water conditions and subsequently released in the estuary, impacting the solute budget of Mo to the sea.In the estuaries, under low salinity conditions, DMo is sequestered onto particles during pre-monsoonal season. The DMo sequestration in the estuary is estimated to be ~ 2 mol day− 1 in the Nethravati estuary and ~ 1.9 mol day− 1 in the Mandovi estuary. During this season sequestration in the estuary is higher than the riverine supply, indicating the sequestration of both marine and river borne DMo. However, the mechanisms involved in the removal process are different in these estuaries viz. oxidative adsorption process in the Nethravati-Gurupur estuary and microbial utilization in the Mandovi estuary. The lower salinity region during monsoon and post-monsoon season shows slight excess of DMo, river borne particulate Mo could release up to 3 to 4 nmol L− 1 by desorption under alkaline higher ionic strength conditions. At higher salinity (> 20 psu) in both the estuaries and in all the seasons, DMo gain is systematic (~ 1 to 37 nmol L− 1). Mo release from river borne particles could contribute only up to 3 to 4 nmol L− 1, which is not sufficient to balance the observed Mo excess. On the other hand, the reductive Mo remobilization from bottom sediments (Mo = 4 mg kg− 1) during sediment diagenesis and subsequent tidal activity, release up to 28 nmol L− 1 of DMo to the estuarine water. Mo release to water column is supported by the gradual enrichment of DMo with depth in the estuary. Therefore, diagenetic release of DMo forms the potential source of DMo excess in the estuary.