Results

GROUNDWATER QUALITY

Trends in the chloride and nitrate concentrations in groundwater were evaluated for the period 1984–98. Because there are few wells in the region specifically for monitoring groundwater quality, most samples are collected from water-supply wells. This leads to some problems in interpreting the data. Whenever the quality of the pumped water does not meet certain criteria, the water is not useful and pumpage is interrupted. The result is that high values are excluded, and a bias in interpretation could result.

In addition, an average of approximately 1 MCM is pumped from all production wells for each liter that is sampled. Thus, inference method (statis-tics combined with general hydrology and knowledge of local conditions) must be used for data analysis, including trends. It is obvious that any estimation is, to large extent, subjective. If the trend at a particular site is expressed as a linear relationship of concentration versus time, there are two conflicting considerations for the length of the period. For a longer period, the results are based on more data and therefore are more robust; but because the trend is the average change within the period, important short-term fluctuations might not be recognized. The adopted period for analysis is a compromise between these considerations.

Chloride and nitrate are common dissolved ionic constituents in water. Chloride is a major component of salinity; thus, trends in chloride concen-tration can be considered to represent the direction, but not the magnitude, of trends in salinity. The principal processes that can cause an increase in chloride concentration are:

1. Infiltration of excess irrigation water. (Most irri-gation water is transpired or evaporates, but the dissolved minerals infiltrate to aquifers.)
2. Anthropogenic activities at the land surface, such as application of fertilizers or pesticides, and land disposal of domestic wastewater or industrial wastes.
3. Seawater intrusion.
4. Artificial recharge of imported water with higher salinity than the native water.
5. Lateral movement of saline groundwater from upgradient areas of the aquifer, or upward move-ment from connected aquifers.

As pumping increases, many of these processes can become more significant, particularly seawater intrusion and lateral movement, which respond to pressure gradients in the aquifer.

Natural concentrations of nitrate in groundwater are low, except in aquifers affected by certain types of evaporite deposits. The sources of most nitrates in groundwater are anthropogenic, such as fertilizer application and sewage disposal. According to the World Health Organization stan-dard, which applies to most of the region, the concentration of nitrate in drinking water must not exceed 45 mg(NO 3 )/L. The Palestinian standard is 50 mg/L and can be increased to 70 mg/L if no alternative water supplies are available.