| The first round of survey and evaluation of groundwater contamination was carried out nationwide in succession. Natural background level of regional groundwater was the key problem needed to be solved in the evaluation firstly, as high levels of concentration were caused by high natural background levels in groundwater while not by anthropogenic activities. Research on natural background levels of groundwater was preferred in order to evaluate groundwater contamination rationally and scientifically. Pearl River Delta was the lead and principal in the social and economic development of Guangdong Province, and was one of the three economic units with fastest-growing economy in coastland of our country. Rapid development of economy resulted in environmental contamination, and groundwater quality was influenced to some degree. Study on natural background levels of groundwater in this area was in virgin state due to the absence of systematical groundwater quality data. Based on comparison of methods home and abroad, 28 indexes such as Cl-,SO42-,HCO3-,NO3-,F-,I-,NH4+,K+,Ca2+,TFe,Mn,As,Cd,TCr,Pb,Cu natural background levels of shallow groundwater in Pearl River Delta were caculated with statistical software SPSS and numerical recipes software Matlab by using of mathematical statistics and trend surface analysis comprehensively, and the spatial distribution regularities and the controlling factors in this region were discussed. The regularity of macrocompositions in Pearl River Delta is affected by topography, rock-soil property, meteorological and hydrologic conditions, regional hydrodynamic conditions, in addition to physical and chemical properties of the elements; valence-alternate elments such as Fe,Mn,As,Cr are mainly controlled by redox conditions; Cl-,SO42-,K+,Na+,Ca2+,Mg2+ are influenced by the mixing of salt and fresh water seawater in coastland; East River groundwater flow system was characterized by horizontal zoning followed by the topography(hills-platform-plain), while West and North River groundwater flow system was relatively complex, though macrocompositions such as Cl-,SO42-,vary regularly according to topography(hills-platform-plain); Natural background levels in this region differ in time scale, and different features are presented in plain terrain and hilly area. The content of Fe, Mn, As decreased, and the content of Hg remained stable in the plain while NO3- and TDS increased with time, which were closely related with human reconstruction activities and in addition, seawater intrusion was the main factor which caused the increase of TDS. The content of Fe in hilly area decreased, while TDS increased within a small range. The result supplies scientific references for the evaluation of groundwater contamination and variation in Pearl River Delta.
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