Inorganic arsenic has been widely used during the production cycle of fertilizers and pesticides and leakage into subsoils and shallow groundwater has been reported in the recent literature. Despite of this, most of the scientific papers have focused on natural arsenic mobilization and geochemical cycling in the subsurface. Since the arsenic mobilization is usually driven by redox processes, pH changes and ionic strength of the water, it is of foremost importance to study its behavior in coastal aquifers where all of these factors essentially coincide. The study site is a former fertilizer plant located near the shore line in Italy, where a pump and treat remediation scheme has been active since 2001. A detailed characterization of arsenic in groundwater was performed to elucidate the mobilization/immobilization mechanisms actually occurring at the site. It was found that most of the arsenic was entrapped in the sediments located near to the source zone, while phosphorous (mainly phosphate) migrated further downstream. A marked vertical reverse trend of EC and Eh was found within the aquifer: the elevated EC was due to the seawater wedge intrusion enhanced by pumping wells, while anoxic conditions were due to organic matter oxidation by sulphate provided by seawater. The arsenic speciation in groundwater showed the net predominance of As(III) except in the upper portion of the aquifer, where post-oxic condition occurred due to continuous oxygen exchange with the vadose zone. In the deeper portion of the aquifer the As(III) concentrations increased up to 900 μg/l, while As(V) remained stable. This feature was probably due to the elevated ionic strength of deep groundwater, which can displace weakly bonded oxianions from exchange sites and also by reducing conditions which promoted As(III) mobilization. Phosphate was present almost only in groundwater due to the reducing conditions of the aquifer. This characterization confirmed that two different mechanisms for arsenic fate were active: (i) the oxidation and consequent sequestration in the upper oxidized zone, near the water table, with a marked decrease of total arsenic in groundwater with respect to the aquifer bottom, where (ii) the reduced and salinized conditions gave rise to very high arsenic concentrations with As(III) as the main groundwater species.
Coastal aquifer contamination by arsenic and phosphorous from a fertilizer production plant
MASTROCICCO, Micol;COLOMBANI, Nicolo';
2012
Abstract
Inorganic arsenic has been widely used during the production cycle of fertilizers and pesticides and leakage into subsoils and shallow groundwater has been reported in the recent literature. Despite of this, most of the scientific papers have focused on natural arsenic mobilization and geochemical cycling in the subsurface. Since the arsenic mobilization is usually driven by redox processes, pH changes and ionic strength of the water, it is of foremost importance to study its behavior in coastal aquifers where all of these factors essentially coincide. The study site is a former fertilizer plant located near the shore line in Italy, where a pump and treat remediation scheme has been active since 2001. A detailed characterization of arsenic in groundwater was performed to elucidate the mobilization/immobilization mechanisms actually occurring at the site. It was found that most of the arsenic was entrapped in the sediments located near to the source zone, while phosphorous (mainly phosphate) migrated further downstream. A marked vertical reverse trend of EC and Eh was found within the aquifer: the elevated EC was due to the seawater wedge intrusion enhanced by pumping wells, while anoxic conditions were due to organic matter oxidation by sulphate provided by seawater. The arsenic speciation in groundwater showed the net predominance of As(III) except in the upper portion of the aquifer, where post-oxic condition occurred due to continuous oxygen exchange with the vadose zone. In the deeper portion of the aquifer the As(III) concentrations increased up to 900 μg/l, while As(V) remained stable. This feature was probably due to the elevated ionic strength of deep groundwater, which can displace weakly bonded oxianions from exchange sites and also by reducing conditions which promoted As(III) mobilization. Phosphate was present almost only in groundwater due to the reducing conditions of the aquifer. This characterization confirmed that two different mechanisms for arsenic fate were active: (i) the oxidation and consequent sequestration in the upper oxidized zone, near the water table, with a marked decrease of total arsenic in groundwater with respect to the aquifer bottom, where (ii) the reduced and salinized conditions gave rise to very high arsenic concentrations with As(III) as the main groundwater species.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
