Jen O’Reilly
October 18, 2001
Coastal Management
Coastal Groundwater Problems
Occurrence of Mercury in Coastal Aquifers of Tuscany, Italy
Coastal area groundwater and aquifer systems are easily prone to pollution and
contamination, especially with an increase in development in the coastal areas. The
aquifers of these areas may be contaminated as a result of seepage of pollutants through
the soils and into the groundwater. Some pollutants of this type include septic tank
systems and fertilizer runoff. Heavily developed coastal areas increase the demand for
water from the aquifer, which results in the phenomena of salt-water intrusion. As a
result of the ease of contamination of these aquifers, wells are often monitored for water
quality, especially in those aquifers that are used as a potable water supply. In February
of 1998, along the southern coast of Tuscany, the routine analysis of well water revealed
irregular mercury contents of the water that were far above the 1g/l standard that was set
by the Italian law (Protano et al., 2000). As a result, several wells in the Orbetello-Mt.
Argentario area were closed. This was not the first case of mercury being found in the
well water. Three years earlier, about 50km further north, in the Gulf of Follonica, three
drinking wells were closed because of mercury contamination. It was noted that in both
of these cases, there was an increase of chloride values due to the intrusion of seawater
(Protano et al., 2000).
The Orbetello-Mt. Argentario region contained a number of main active pumping
wells that contributed to the drinking water supply of the area. Mercury contamination
was found in several of these wells. Figure 1 shows a map of the area, as well as the
Figure 1
contaminated and uncontaminated pumping wells. The figure also shows the inland
advance of the seawater into the groundwater system. Overpumping of the wells has
caused salt-water intrusion to occur at a rate faster than natural freshwater can replenish
the aquifer. (Protano et al., 2000) An increased demand is placed on the aquifer during
the summer months, not only because of droughts, but also because this area is a popular
summertime seaside resort. As a result, there is a seasonal variation of high water and
low water conditions in the aquifer. (Grassi and Netti, 2000)
During the spring and the summer months of 1998, extensive sampling of the
groundwater and coastal marine water was carried out (Protano et al., 2000). Grassi and
Netti (2000) report the findings of a study carried out from June to October of 1998 by
the Tuscan Regional Environmental Agency and the International Institute for
Geothermal Research of the Italian National Research Council.
The results from the study by Protano et al. (2000) indicated that water filtration
was able to eliminate most of the mercury, so the occurrence of mercury in the aquifer is
somehow in a particulate phase. The immediate hypothesis of an anthropogenic input of
mercury into the groundwater is voided by the fact that the mercury occurs over a large
area and that the contaminated wells are located within protected areas. From this
conclusion, a natural input of mercury must be occurring. Additional hypotheses of a
diffuse degassing phenomenon from active hydrothermal circulation as well as a
hypothesis of leaching of mercury from the aquifer rocks were refuted with additional
testing. In particular, the aquifer was known to have a very low concentration of mercury
in its rocks. The main mercury-bearing mineral is cinnabar, which is known to be quite
stable in low oxygen environments. (Protano et al., 2000)
Protano et al. (2000) believes that a simple mechanism of gaseous mercury
generation occurs when the seawater enters a medium that is separated from the
atmosphere. They believe that the mercury may come from contaminated wastes from
the area’s history of mining and roasting of mercury ores. Over time, the wastes may
have reached the sea and formed chlorocomplexes in the salt water. These complexes are
sensitive to redox reactions that may take place in oxygen poor root zones of aquifers and
create the gaseous mercury. (Protano et al., 2000)
Grassi and Netti (2000) indicate that in addition to the occurrence of mercury as
ore bodies, it also occurs as clastic material eroded from primary mineralisations and
deposited along alluvial valleys. Studies of the geochemistry properties of the tested
wells indicated that the salinity of the groundwater was strongly influenced by the
chloride concentration from the marine waters. It was concluded that the seawater
causing the salinisation of the aquifers was also the cause of mercury mobilization. It
was found that increasing chloride concentrations was coupled with increasing mercury
concentration (Figure 2). The stability of naturally occurring mercury within the aquifer
is strongly dependent upon the pH, redox conditions and geothermal conditions of the
fluids. Mobilization of mercury occurs by the formation of complexes with chloride,
which can greatly increase that amount of dissolved mercury in solution. (Grassi and
Netti, 2000)
Figure 2
The view by Protano et al. (2000) that mercury is from a source outside of the
aquifer contrasts that of Grassi and Netti (2000) who propose that the mercury originated
from within the aquifer. As long as the chloride content was low the amounts of mercury
released into solution was too small to be detected. With increased exploitation of the
groundwater and the resulting intrusion of salt water, the mercury in solution has
increased due to the interaction with chloride from seawater. The local differences in
mercury concentrations depend upon the amounts of naturally occurring mercury in an
area, as well as the pH and redox states of the water (Grassi and Netti, 2000).
Although these two studies give two different sources of the mercury that is
contaminating the aquifer wells, the method of contamination of the wells is the same, by
salt-water intrusion. The management agency in this area has two issues to deal with, the
common worldwide problem of saline waters entering groundwater, as well as the
problem of natural mercury contamination. In a study conducted by Melamed and Villas
Boas (2001), physico-chemical additions were tested to counteract mercury pollution that
occurred from gold recovery practices, however, most of these tests were conducted on
soils. It may be possible to develop an additive to place into the aquifer systems that
would reduce the amount of mercury in solution, and at the same time, would pose no
new contamination risks. Although such a solution would be difficult to finalize, the
fundamental problem of saltwater intrusion would not be addressed. Perhaps the best
solution for the Tuscany area would be to reduce the severity of the marine water
intrusion. G.H.P. Oude Essink (2001) proposes several methods to compensate for and to
control the salinisation process; however, he indicates that such measures will take a
great amount of time, from decades to centuries. The countermeasures shown in Figure 3
include: (1) freshwater injection; (2) extraction of saline and brackish waters; (3)
modifying pumping practices, such as lowering rates of extraction, relocation of
extraction wells, and usage of shallow water wells; (4) land reclamation; (5) increase of
the upland recharge areas; and (6) creation of physical barriers, such as sheet piles, clay
trenches, and chemical injections. Some of these countermeasures may have
environmental consequences as well as economic limitations.
Figure 3
Works Cited
Grassi, S., Netti, R., 2000. Sea water intrusion and mercury pollution of some coastal
aquifers in the province of Grosseto (Southern Tuscany – Italy). Journal of
Hydrology 237, 198 – 211.
Melamed, R., Villas Boas, R.C., 2000. Application of physico-chemical amendments for
the counteraction of mercury pollution. The Science of the Total Environment
261, 203 – 209.
Oude Essink, Gualbert H.P., 2001. Improving fresh groundwater supply – problems and
solutions. Ocean & Coastal Management 44, 429 – 449.
Protano, G., Riccobono, F., Sabatini, G., 2000. Does salt water intrusion constitute a
mercury contamination risk for coastal fresh water aquifers?. Environmental
Pollution 110, 451 – 458.