A two-column reactor was designed to remove dissolved As and Cd from comtaminated water. The reactor functions by equilibrating the targered water with CO2 and directing it via saturated flow through acolumn of crushed siderite. This results in siderite dissolution and an increase in dissolved Fe(Ⅱ). The feedwater is them directed into the top of a second, aerated cloumn of crushed limestone, where it passes by unsaturated flow. The Fe2+ ion oxidizes quickly to Fe3+ and precipitates as Fe(Ⅲ) oxyhydroxide, which is an effective sorbent of AsO34-. The aeration that occurs in the second column also removes dissolved CO2 from the feedwater. This causes precipitation of Ca and Cd carbonates. Together, the processes reduce As and Cd concentrations from 1 and 3mg/1, respectively, to below detection (respectively<0.005and <0.01mg/1). A time-limited reduction in Cr conentration also occurred. Much of the As was removed in the first column of the reactor, because Fe(Ⅲ) oxyhydroxides also formed there. This was due to oxidation of Fe(Ⅱ) by Cr(Ⅵ) and other oxidants present on the input wastewater. Although As is removed in the reactor columns by a sorption mechanism, the sorbent responsible, Fe(Ⅲ) oxyhydroxide, is continuously produced during the operation of the reactor. Thus, unlike attenuation in a system with a fixed amount of sorbent, breakthrough of the As contaminant should never occur.

The response of carbonate aquifer to climate changes and ground water exploitation was studied at Shentou Springs. The springs consist of about 100 spring points. The long-term (1958-1999) annual average discharge of the springs is 6.86m3/s. The discharge has diminished since the early 1960s. In 1993, the discharge (3.81m3/s) was the smallest during the period of record. Data of restored spring discharge and precipitation during 1958-1999 were analyzed using a seasonal decomposition method. The result of analysis shows that the discharge of Shentou Springs responds remarkably to precipitation change. Lowering of ground water table and attenuation of spring discharge at Shentou in the last several decades are largely a response of the ground water system to the deficiency in regional precipitation in northern China. As compared with the effect of climate, human activities (mostly in the form of ground water abstraction) are secondary in affecting spring discharge. When the volume of exploited Quaternary ground waters that are partly recharged by karst waters exceeds 6400

Groundwater is the most important source of water supply in Datong city. However, the levels of shallow Quaternary groundwaters from urbanized areas have been declining continuously and groundwater quality deteriorating in recent years. Understanding the geochemical evolution of groundwater is important for sustainable development of the water resources in Datong. Mineral hydrolysis of alumino-silicate minerals such as plagioclase and clinopyroxene, is the primary process controlling the concentration of H4SiO4 in the study area. Speciation calculations using the geochemical modeling code PHREEQC indicated that hydrolysis of bedrock, mainly composed of asalt and metamorphic rocks, is the major hydrogeochemical process controlling groundwater chemistry. The study area can be divided into 3 hydrogeochemical zones: A. Recharge (unimpacted) zone, B. Intermediate (industry-impacted) zone, and C. Discharge (agriculture-impacted) zone. Ion exchange and industrial and/or agricultural contamination contribute to the increase of Na+ from Zone A to Zone C, where the concentration of NO3-is up to 461.5mg/l with a mean value of 101.5mg/l, indicating that agricultural practice seriously affects groundwater. Sulfate concentration in groundwaters in an alluvial fan at Datong is extremely high, up to 1172.9mg/l, and shows a close relationship with the concentrations of trace elements, especially Ni and Co, indicating that coal mining is the main contamination source for groundwater from the alluvial fan, in addition to agricultural activities.

A two-column limestone reactor has been designed to reduce fluoride concentrations from wastewaters to below the maximum contaminant level (MCL limit) of 4mg/L. The reactor functions by forcing calcite (CaCO3) to dissolve and fluorite (CaF2) to precipitate in the first column. The second column is not necessary to remove fluoride but is used to precipitate the calcite dissolved in the first column. This returns the treated water to its approximate initial composition. In laboratory experiments, the fluoride concentration of the effluent from feedwaters containing initial F amounts of up to 109mg/L were brought below the MCL limit of 4mg/L with a porewater residence time within the column of 2h. Profile sampling shows that fluoride is reduced from 109 to 8mg/L after only 35min within the reactor. The major advantage of this potential technology over existing liming and ion exchange methods is that system monitoring is minimal, regular column regeneration is not required, and chemicals are not permanently added to the water. Because the CaCO3 dissolved in the first column is precipitated in the second, the reactor has potential application to reduce the concentrations from wastewaters of contaminants similar in charge and size to Ca2+ and CO32- through coprecipitation reactions. In a pilot experiment where fly ash leachate was spiked with mg/L levels of Cd, As, Cr, and Se and directed through the reactor, reductions in all elements except Cr occurred. Cd was the most notable. Influent concentrations from 2 to 30mg/L were reduced to below detection (<0.01mg/L)