Speciation of Selenium in a Nile Delta Lagoon and SE Mediterranean Sea Mixing Zone

Abstract

Dissolved selenium species have been measured in Mex Bay located west of Alexandria, which receives intermittently oxic/suboxic water from the Mariut Lagoon. Total dissolved selenium concentrations in the lagoon reached 12·02nMkg−1during the summer, with Se(IV) as the predominant species constituting about 38%. Total dissolved Se behaved conservatively upon mixing with the bay water. Selenite (range 0·13–3·9nMkg−1and 0·09–1·9nMkg−1) and selenate (range 0·89–2·6nMkg−1and 0·63–1·6nMkg−1) during high and low discharge conditions correlate linearly with salinity. During high flow, selenite predominates below salinity 26. Se(IV) (38–47%) and organic Se (37–40%) dominated under suboxic conditions in the Mariut Lagoon, while Se(VI) (67–72%) was predominant in the oxygenated Mex Bay waters followed in abundance by organic Se (20–22%) and Se(IV) (8–11%). Selenium behaviour in the bay seemed more related to phosphorus than silicon; all are simply diluted when brackish lagoon water mixes with neritic Mediterranean waters. The absence of selenium reactivity is caused by the short residence time (28 days) of the bay water.About 1500kgyear−1of total dissolved selenium reaches the bay from the Mariut Lagoon with the peak during July–October. This constitutes 58% of the total dissolved Se flux to the SE Mediterranean derived from landbased sources. Selenium residence time in Mex Bay is 12·6 days, while its assimilative capacity for Se is 5515kgyear−1μgl−1. About a 40% reduction in the present day discharge from Mariut Lagoon will decrease selenium levels in the bay water to safe limits.