Results

GROUNDWATER QUALITY

COASTAL AQUIFER—CHLORIDE

Figure 10. Map showing trends for chloride concentrations in groundwater of the Coastal aquifer during 1984–98.

Figure 11. Graphs showing mean annual concentrations of chloride for selected locations in the Coastal aquifer during 1984–98.

Degradation of water quality is the main problem facing water management of the Coastal aquifer. Trends in chloride concentration in the aquifer during 1984–98 are shown in figure 10. Concentrations in samples from most wells throughout the aquifer were upward or were about constant during the period. Upward trends were identified for 59 percent of the wells included in the anal-ysis, and downward trends were identified for only 7 percent. No major spatial patterns were obvious except for the distribution of the largest increases, which seem more prominent in the central and southern parts of the aquifer. Localized pumping may superimpose “noise” on regional patterns and trends. Time-series plots of chloride concentrations in a few locations (fig. 11) illustrate some of these processes.

IHS well 21214302 (WQ–1) is in an agricultural area, and the increase in chloride concentration probably results from infiltration of minerals applied in irrigation water. Most of the increase occurred in 1992 following a year of high precipitation. Recharge due to this precipitation might have mobilized chloride that had been accumulating in the unsaturated zone for many years.

IHS wells 20313903 (WQ–2A) and 20313901 (WQ–2B) are a pair of water-supply wells in the same location, but the wells draw water from different depths in the aquifer. The trend in chloride concentration for well WQ–2A was -18 mg/L per year, but most of this decline occurred after 1991 when the well was taken out of production because of high salinity. Production from well WQ–2B was increased at the same time, and the chloride concentration increased due to the local depression in water level.

IHS well 14212202 (WQ–3) is in an urban area between Tel Aviv and Ashdod. The increase in chloride is similar to that in well WQ–2B but starts at a lower concentration.

Water quality in IHS well 10711501 (WQ–4) is affected by industrial contamination. Chloride concentration exceeded 800 mg/L and increased 39 mg/L per year during 1984–98. In contrast to other wells, chloride decreased immediately following the high precipitation of 1992, probably because the contamination was diluted. However, this effect lasted only 2 years, after which chloride increased sharply.

PWA well A/95 (WQ–5) is near the Mediterranean coast and is affected by seawater intrusion, resulting in a fairly steady increase in chloride concentration of 15 mg/L per year during 1984–98.

PWA well F/68A (WQ–6) is located near Wadi Gaza south of Gaza City. Water quality in this area is affected by disposal of domestic wastewater. Chloride concentrations in well WQ–6 increased steadily at a rate of 45 mg/L per year during 1984–98.

In the southwestern part of the Coastal aquifer, wells are affected by lateral inflow of saline groundwater from the east (upgradient). Chloride concentrations in PWA well N/16 (WQ–7) were high but relatively constant during 1984–90, then increased greatly from 1990 to 1998.

A.	B.
Figure 12. Maps showing chloride concentrations in groundwater of the Coastal aquifer in (A) 1998 and (B) 1934.

Even though trends were fairly uniform throughout the aquifer, chloride concentrations were much higher in the south—for example, in wells WQ–4, WQ–6, and WQ–7. A map of concen-tration isopleths for 1998 is shown in figure 12A. High concentrations did not affect the northern part of the aquifer and occurred only in isolated locations in the central part. However, high concentrations were more general in the south. A similar pattern appears in isopleths of chloride concentration from samples collected during 1934 (PWA data base), prior to much of the urban and industrial development in the Coastal Basin (fig. 12B). A source of saline water, probably naturally occurring, is clearly indicated in the southeastern part of the aquifer. This is likely the same source that affected downgradient wells, due to lateral movement, during 1984–98.