HYDROLOGIC SETTING

The climate of the study area is semi-arid, with a mean annual rainfall of about 520 mm. About 70%

of the precipitation occurs between July and September. The mean annual temperature is about 13°C and the temperature difference between summer and winter is large. The potential mean annual evaporation is 1200 mm.

Shijiazhuang City, about 72 km², is located in the Hutuo river alluvial fan, east of the Taihang Mountains (Fig. 1). The plain slopes gently to the east from about 100 m to about 50 m above sea level. The topographic gradient is smooth, about 4‰ in the west and 2‰ in the east.

The underlying quaternary deposits are divided into 4 aquifers (I, II, III and IV, Fig. 2). The shallow aquifers, I and II, are composed mainly coarse sand and gravel with total thickness of 34–70 m and are extensively pumped. Aquifer I has been almost completely dewatered; as a consequence, aquifer II is presently (2001) the main aquifer pumped water supply. The deep aquifers III and IV are composed mainly of fine sand. A continuous aquitard separates these aquifers from the overlying aquifers and the groundwater is confined. Recharge to the deeper aquifers is limited and they have not been extensively pumped for water supply.

Under predevelopment conditions (50 years ago), groundwater recharge was through infiltration of precipitation and lateral inflow from the mountain front to the west. Discharge was mainly through lateral outflow to the east. With the development of an irrigation canal system, irrigation return flow has become another recharge source. At present (2001), groundwater pumping is the main method of discharge, smaller amounts of water discharge through lateral outflow to the east.

2.1. Groundwater pumping and water table drawdown

In 1959, the depth of groundwater underlying Shijiazhuang City was between 1 m to 5 m and groundwater flowed mainly from northwest to southeast (Fig. 3). Beginning in the 1970s, about 3.46 × 10⁸ m³ groundwater has been extracted annually from aquifers underlying Shijiazhuang City.

And a large cone of depression has developed in the regional water table. Groundwater levels have decreased about 1 meter per year in the centre of the cone of depression (Appendix I, Fig. 4). By 2000, the depth to groundwater was from 30 m to 40 m in the centre of the pumping depression and the groundwater flowed mainly from periphery towards the centre of the pumping depression (Fig. 5).

2.2. Sewage utilization and discharge

The discharge of sewage and industrial wastewater increased yearly with increasing population, industrial growth, and pumping of groundwater. The total discharge in 2000, was more than 540 000 m³ per day for industrial wastewater and 200 000 m³ per day for urban sewage.

Before 1970, there was no sewage discharge system in residential areas in Shijiazhuang city. Even at present, most of sewage is discharged through open ditches without liners to prevent leakage. The main sewage ditches are East open ditch and West open ditch in south part of the city. The two ditches join and then discharge to the Xiaoxiaohe River south of the city. Sewage not discharged to the river is used for irrigation without treatment, south of Shijiazhuang City.

FIG. 2. The hydrogeological cross section of Shijiazhuang City. 1. clay; 2. clayey soil; 3. clay contained gravel; 4. sand and gravel; 5. sand; 6. boundary line of aquifer; 7. boundary line of stratum.

FIG. 3. Water levels and depths to groundwater, Shijiazhuang City, China, 1959.

20 30 40 50 60 70

1965 1970 1975 1980 1985 1990 1995 2000

Time (Year)

Water table elevation (m)

FIG. 4. Groundwater levels in the centre of the pumping depression underlying, Shijiazhuang City, China.

FIG. 5. Water levels and depths to groundwater, Shijiazhuang City, China, 2000.

2.3. Predevelopment and 1992 major-ion chemistry

From 1958 to 1959, 21 wells were drilled for water supply in Shijiazhuang City. Groundwater samples from each well were analyzed for pH, major ions, nitrate, hardness and dissolved solids (Appendix II).

These are the earliest groundwater chemical data in the study area and they reflect background groundwater chemistry without (or with only mild) pollution. At that time, most of groundwater had TDS values around 300 mg/L, and only one sample had a TDS concentration greater than 600 mg/L.

Hardness of most groundwater samples was less than 300 mg/L and only one sample had a value over 400 mg/L. Water chemical types were mainly HCO₃–Ca and HCO₃–CaMg (Fig. 7).

Groundwater chemical concentrations have increased annually since 1959. The concentrations of groundwater constituents in 1992, the latest year for which comprehensive chemical data are available, are listed in Appendix III. At that time average concentrations for selected constituents were:

dissolved solid = 710 mg/L; hardness = 502 mg/L; Cl^– = 118 mg/L; and SO# = 163 mg/L.

Concentrations from selected well that were sampled as part of this study in 2001 are given in Appendix IV. Concentrations of measured constituents were higher in sewage irrigated area than in areas that were not irrigated with sewage (Fig. 7), and water chemical types had changed since 1959 (Fig. 8). Some of the increase in calcium, magnesium, bicarbonate, and sodium concentrations has been attributed to increased dissolution of minerals within the unsaturated zone and subsequent cation exchange as a result of water table decline (Appendix V). These reactions are driven by high partial pressures of CO2 in the unsaturated zone atmosphere resulting from oxidation of organic carbon in sewage used for irrigation. In addition, trace pollutants from industrial activity, such as Cr⁶⁺, phenol and CN^–, were found in groundwater in high concentrations.

2.4. Predevelopment and present-day (2001) nitrate concentrations

In 1959, water from most sampled wells in Shijiazhuang City had nitrate concentrations less than 2 mg/L, with a maximum concentration of 6.6 mg/L (Appendix II). By 1978, the average nitrate concentration was 15.7 mg/L. Since 1978, groundwater quality monitoring has been done routinely in about 90 wells in the city. Concentrations of nitrate increased steadily in water from these wells between 1978 and 2000 (Fig. 9). According to the data, by 1996, the average nitrate concentration was 44.8 mg/L. Histograms showing nitrate concentrations in water from wells in Shijiazhuang City between 1980 and 2001 show a consistent shift to higher concentrations and a marked decrease in the number of wells yielding lower nitrate concentrations (Fig. 10). Nitrate concentrations were higher in areas irrigated with sewage than in non-sewer irrigated areas (Fig. 7) and in general nitrate

concentrations are lower in the northern part of the study area and higher in the southern part of the study area (Fig. 11). As previously discussed, sewage from disposal ditches is commonly used for irrigation in the southern part of the study area.

Water from wells in the vicinity of borehole B03 in the area of high nitrate contamination south of Shijiazhuang City has been contaminated by nitrate since 1982. Sewage irrigation started in this area in 1965 and stopped in 1980. Only 15 years elapsed between sewage irrigation and the onset of groundwater nitrate contamination in this area. In 1982, the groundwater table depth was around 14 m.

and nitrate must have moved downward at a rate of more than 0.9 m/y. Assuming an average volumetric water content of 23%, the recharge rate must be at least 200 mm/y. This calculation probably greatly underestimates the actual recharge rate since a large amount of high-nitrate water would have had to accumulate near the top of the water table before contaminating wells.