Distribution of arsenic and other minor trace elements in the groundwater of Ischia Island (southern Italy)
The island of Ischia belongs to the active volcanic area of Naples. It is formed from Quaternary volcanic rocks and exhibits intense hydrothermal activity, which is manifested through numerous springs, fumaroles and sporadic geysers. The content of minor and trace elements in groundwater has been analyzed, including some elements that are considered toxic for humans. Mean concentrations of As, B, Fe, Mn, Sb, and Se in samples from 43 aquifer points exceed the WHO recommended values and the limits set by European and Italian legislation (98/83/CE and DM 471, respectively). In general, the spatial distribution of the elements follows a common pattern: it is governed by a marked structural control, which favors hydrochemical processes that liberate the elements into the water.
KeywordsGroundwater Minor and trace elements Hazardous elements Health laws Isocontent maps
Elements such as Co, Cr, Cu, Fe, Mn, V, Se and Zn are important for the proper physical development of the human body, and are defined as essential micronutrients (Coppes 1999), despite the fact that some of them can be extremely toxic. Others, such as As, Ba, Bi, Cd, do not appear to be essential to human metabolism (WHO 1996). If present in elevated quantities, both groups are associated with morphological abnormalities, growth disorders, cancer, and even death (WHO 1996).
The Chaco-Pampean Plain (Argentina), Antofagosta (Chile), the Tumet and Tianshan Plains (China), and the West Bengal region are only some of the numerous sites where As concentrations in the groundwater are higher than the WHO recommended value. Seven out of sixteen districts of West Bengal have been reported to have groundwater As concentrations above 50 ppb; the total population in these districts is over 34 million (Mandal and others 1996).
In groundwater, an enrichment in As and other associated toxic trace elements, such as Sb, V, Sr, B, Fe, Cr, Ba, and Co, can be due as much to natural phenomena as to anthropogenic activity. Nevertheless, it appears that As has an essentially natural origin. All of the elements listed give rise to problems of potability and are denominated by some authors as natural contaminants, either transitory or permanent (Nicolli and others 1989).
In 1993, WHO decided to reduce the maximum recommended limit for As from 50 to 10 ppb in water destined for human consumption. This was a consequence of a greater understanding of its toxicity and of the carcinogenic affects associated with it; nonetheless, some authors believe that As can be tolerated up to 50 ppb (Smedley and Kinniburgh 2002). In the United States, As is high on the list of toxic substances compiled by the ATSDR (ATSDR 2000), and in January 2001 the EPA adopted a limit value of 10 ppb (although a value of 50 ppb will be permitted until the 23rd January 2006).
In the countries of the European Union, the quality parameters for water destined for human consumption are set by Directive 98/83/CE; following the WHO recommendations, this directive limits the concentration of As (10 ppb) and of other elements considered harmful to public health.
In 1999, the Italian government incorporated the limits into its national legislation, by means of Ministerial Decree nº471 (Ministero dell’Ambiente 1999); this lays down maximum admissible concentrations for certain toxic trace metals in groundwater, and any groundwater with concentrations exceeding these are considered to be contaminated.
Concentrations of As in natural water varies from 0.5 to more than 5000 ppb (Smedley and Kinniburgh 2002), although in fresh, unpolluted groundwater, it is usually of the order of 1–10 ppb. In Italian volcanic systems, the As content oscillates between 0.1 and 6940 ppb, with the highest concentrations occurring where hydrothermal phenomena are active in shallow groundwater zones (Aiuppa and others 2003).
Inorganic As is classified as cancerigenic, although it is in its volatile form (AsH3) that it possesses the greatest toxic risk (EPA 1999). The main compounds of As in water are inorganic, and the As in these generally occurs in two oxidation states: pentavalent (arsenate) and trivalent As (arsenite). Additionally, the formation of volatile compounds by methylation has been described in natural systems for S, Ge, As, Se, Cd, Sn, and Sb.
However all forms of As are potentially harmful to human health and the inorganic species are more toxic than pentavalent methylated arsenic compounds. In contrast to other trace elements (Se, Sb, Mo, V, Cr, Pb and others), As is the only one that is relatively mobile under oxidizing and reducing conditions, within the range of pH commonly found in groundwater (6.5–8.5). Other toxic trace elements are found in solution as cations (Pb2+, Cu2+, and so on) but they become more and more insoluble as pH increases (Smedley and Kinniburgh 2002).
Volcanic gases, geothermal fluids and degasification phenomena can all be natural sources of all of these elements. Due to their high temperatures, geothermal waters are able to dissolve relatively high amounts of metal(loid)s out of their parent rocks (Hirner and others 1998).
The objectives of this study were to quantify the natural concentration of As and other minor and trace elements, determine their spatial distributions within the study area, and extend knowledge about the geochemistry of the groundwater of the island. With these aims in mind, 43 aquifer points were surveyed in the western half of Ischia Island.
The island is dominated by Mount Epomeo (787 m a.s.l.), which represents a horst-vulcano-tectonic type structure (Rittmann 1930). There is a series of eruptive centers, which have formed articulated structures and lithologies. The numerous thermal springs, fumaroles and geysers, and to a lesser extent, the seismicity and bradisism indicate that the zone is currently active.
The most intense hydrothermal activity is associated with the faults and fractures that border the horst, above all in the north-eastern and eastern sectors and along the length of the NW-SE regional fault (Rittman 1930; De Gennaro and others 1984; Vezzoli 1988). Alkali-trachytic rocks predominate, with subordinate trachybasalts, latites and phonolites.
According to Celico and others (1999), one can identify two areas with different hydraulics: the first is the graben in the northeast of the island, which is highly transmissive and is recharged by direct infiltration and marine intrusion; the other area is Mount Epomeo and his border zone, which is intensively fractured and consists of permeable materials (loose pyroclasts and detritic layers), semipermeable (tuffs and alluvial deposits) and impermeable materials (marine sediments). The latter zone lies to the northeast; it is more heterogeneous and anisotropic, and less transmissive. Recharge derives from fluids originating at depth and from seawater. The complex tectonics and lithology are reflected in the hydrogeology, which behaves as a multilayer aquifer (Celico and others 1999).
The geothermal system of Ischia has been the subject of several investigations. De Gennaro and others (1984) formulated a geothermal model of the island in which a deep source of fluids was represented by a large magmatic body located at a depth greater than 3000 m and with a temperature superior to 200 °C. Later on, Caparezza and others (1988) suggested the existence of two intermediate systems, and Panichi and others (1992) estimated the temperature of the reservoir in the range of 160–240 °C. Lastly, Inguaggiato and others (2000) reached the conclusion, supported by carbon isotope data, of the existence of a magmatic reservoir containing liquid and gas, in which the dominant liquid has a temperature of 280 °C. However, they don’t discard the possibility of the existence of a second reservoir, as suggested by Tedesco (1996), and located in the eastern part of the island at a depth of more than 4 km. This reservoir is recharged by seawater and high temperature gases, and by low temperature groundwater of marine and meteoric origin (De Gennaro and others 1984; Panichi and others 1992; Inguaggiato and others 2000). The latest research seems to indicate that the fluids come from a single, biphase reservoir lying at depth. This has a dominant liquid phase, which is recharged principally from seawater and gases, both of which are enriched. This fluid interacts with the low-temperature groundwater that originates from marine and meteoric water (Inguaggiato and others 2000).
Sampling and analysis
43 aquifer points were sampled in July 1999, mostly from boreholes less than 100 m deep. The wells were pumped for at least 30 min before the sample was taken, although a daily start-up is necessary due to the impact of tourism on the island. Electrical conductivity, pH, and water temperature were measured in the field. Samples of 100 ml water were taken in double-capped, polyethylene bottles. They were filtered using 0.45 μm Millipore membranes, and acidified to 1% with pure nitric acid. The samples were stored at 4 °C and subsequently analyzed in the laboratory (Acme Analytical Lab. Ltd.) by atomic spectrometry (ICP-MS). 72 elements were determined, of which the following are considered in this study: As, B, Ba, Be, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sb, Se, Tl, V, and Zn.
Statistics and limit values (elements in ppb, EC in mS/cm and T in °C). Underlined values are provisional (WHO 1996)
Data treatment and results
All of the maximum concentrations were recorded from only eight samples (25, 43, 31, 15, 20, 23, 8, 2) and the raw analytical results show that, with the exception of Cd, Co, Cr and Zn, all of the elements studied greatly exceeded the limit values set down by the various agencies. Only in a few cases did the sample size for these elements reached 50% of the 43 aquifer points considered. As and Fe, for example, were detected at 37 and 26 of the 43 aquifer points, respectively. As concentrations were below 10 ppbin only two of the samples (25 and 32).
Spearman coefficients of correlation
It likewise reveals the correlation (>0.60) between Se, B, Mn, Cu, and Fe and, to a lesser extent, with V, Ba and Ni.
Temperature has a significant correlation with Sb and As, while EC is most correlated with Mn, Tl, V, Se and Fe. As is negatively correlated with EC and with V, Zn, Co and pH. As content is linked to Pb, Sb, and to a lesser extent with Tl (0.52) and temperature (0.57). As was detected in 86% of the wells; its mean value was 175.66 ppb, and 95% of the points investigated exceeded 10 ppb.
B was recorded in almost all of the samples, and in 49% of these it exceeded 1000 ppb. B is correlated with Tl, Se, V, Ba, Mn and Cu. The origin of this element within the study area does appear to be exclusively related to seawater (Morell and others, in preparation). Of the metals to which it is correlated, Mn is worthy of note because of the difference between the concentrations set as limit values: only seven samples (16%) exceed the 500 ppb set as a provisional value by WHO, while 65% of the samples exceed the European standard.
One of the possible explanations could be that manganese sulfide dissolves in water containing carbon dioxide, since some of the waters in the zone are classified as of sodium bicarbonate type (Celico and others 1999).
V is correlated with B, Fe, EC, Se and Cu. Its correlation with As is negative. This element was recorded in only 37% of the samples considered, and its mean was 79.19 ppb. There are no limit values set for V in groundwater or potable water; nonetheless, it does play a very important biochemical role, and deficiency is associated with growth problems and hypocholesterolemia (Coppes 1999). A possible origin of this metal is from the clay minerals that result from the alteration of igneous rocks.
Se is correlated with B, Tl, Mn, Cu, Fe, Ba and Ni. It was present in 91% of the samples, and the mean concentration was 24.66 ppb; 41% of samples exceeded the established levels. Its presence in the samples may be due to the fact that it is an element of the volcanic glass that occurs in the region. It is an essential nutrient for animals, and it also acts an agent in the detoxification of certain heavy metals, particularly cadmium. Excessive doses are, however, harmful.
Little information is available about the occurrence of Sb in groundwater. Considered to be a toxic element, it is found in small quantities in the lungs and in hair. This element presents a statistically significant non-zero correlation at the 99% confidence level with temperature and As. The mean (9.26 ppb) was double the maximum legal limit, though only eleven samples actually exceeded this limit.
In order to understand the geographic distribution of the various trace elements, and where concentrations are greater than those recommended by WHO or in excess of European standards, isocontent maps and maps of health risk were drawn, with an interpolation based on an inverse function of distance (I.D.W.), using a 50×50 m grid. If one examines the distribution of the selected elements, bearing in mind the correlations calculated, three areas stand out as having consistently high metal concentrations. Moving from north to south, these are the zone between Punta Caruso, San Montano and Fango; the coastal zone around Citara-Cuotto; and the zone of boreholes lying between Panza, Succhivo and Punta Chiarito. In contrast, the wells around the port of Forio town frequently yield low concentrations of the elements investigated.
Samples containing high concentrations correspond to intensively fractured zones, with recent lithologies that have not been impoverished in terms of their chemical constituents, and in which temperature and electrical conductivity are moderately high (Fig. 2). However, with respect to other volcanic areas of Italy, the measured mean concentration (175.7 ppb) is higher. Aiuppa and others (2003) studied the content of As in the different volcanic areas of southern Italy and found concentrations between 0.1–6940 ppb. With the exception of the Phlegrean Fields (743 ppb), the mean value of this element is lower than 50 ppb, and it seems that the highest concentrations are where hydrothermals act at small depths (Aiuppa and others 2003). In the Phlegrean Fields – an area geologically related to Ischia – Valentino and others (2003) measured up to 6050 ppb in a spring.
In all likelihood, the structural and lithological peculiarities of these zones favor mixing, which augments the concentrations of minor and trace elements in the groundwater.
Aquifer points in the northern and southern areas exhibit temperatures higher than 50 °C, except for sample 22 (47.7 °C), and electrical conductivity is frequently in excess of 9.0 mS/cm.
Temperatures of less than 30 °C and conductivities of between 0.7–14 mS/cm characterize the wells between the port and Pennanova. Concentrations of all the elements examined are all moderately low. Along the coast and inland from the Bay of Citara, the sampling points were more densely spaced and concentrations were moderately high.
The data presented in this paper represent the first evaluation of the natural levels of certain minor and trace elements in groundwater undertaken on the island of Ischia, which can be considered as a test area. Mean concentrations of As, B, Fe, Mn, Sb, and Se from 43 sampling points exceeded both WHO recommendations and the limit values laid down by the European directive 98/83/CE (European Council 1998) and Italian legislation DM nº 471 (Ministero dell’Ambiente 1999). Concentrations of As and Fe exceed the established limits by up to 140 and 1000 times, respectively. However, the number of samples was quite low, and such extreme values may be anomalies that are unrepresentative of the regional trend.
The most abundant trace elements in the groundwaters of Ischia are As, Mn, B, Se and Fe, which were recorded in 86%, 65%, 45% and 35%, respectively, of the samples analyzed.
The distribution maps for the elements are very similar, and this indicates a strong territorial control that favors and accentuates the physico-chemical processes responsible for the observed water chemistry.
The most affected areas correspond to fracture zones that enable the ascent of the hot hydrothermal fluids that encourage water-rock interaction. The mean concentration of As found close to volcanic areas, geologically related to Ischia, is approximately four times larger than the mean value found in the study area.
The analytical data used in this study were obtained using funds from the Department of Vulcanology and Geophysics of the Universitá degli Studi di Napoli “Federico II” as part of my degree thesis, completed in June 2000 and directed by Professor Benedetto de Vivo. I am grateful for his help and advice during this work. My sincere thanks also go to Professor Antonio Pulido Bosch for his comments on the initial manuscript of this paper.
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