Contamination and natural attenuation characteristics of petroleum hydrocarbons in a fractured karst aquifer, North China.

A rare super-large fractured karst aquifer located in Zibo city, Shandong Province of Northern China was polluted by petroleum hydrocarbons from a petrochemical company. Over the last 30 years, it has been the focus of several remediation efforts. In this study, the contamination and natural attenuation characteristics of the petroleum hydrocarbons were elucidated using hydrogeochemical indicators (DO, DOC, Cl(-), HCO3(-), pH, NO3(-), and SO4(2-)), petroleum hydrocarbons elements and environmental isotopes (delta(15)NNO3, delta(18)ONO3, delta(13)CDIC, and delta(13)CDOC). With the aid of GIS, statistical analyses, as well as first-order decay model and electron-acceptor-limited kinetic model, the spatio-temporal evolution characteristics of the petroleum hydrocarbons were modeled. Results showed a positive natural attenuation trend over the last 3 decades where intrinsic biodegradation mechanism was found to be the most important factor driving the degradation of hydrocarbons in the aquifer system. The hydrogeochemical association between the indicators and petroleum hydrocarbons provided the evidences of biodegradation and also served as markers, highlighting the occurrence of anaerobic respiration without methanogenic activities within the heterogenous karst media. Furthermore, the mean natural attenuation rate of petroleum hydrocarbons was calculated to be 3.76 x 10(-3)/day whereby the current highest petroleum hydrocarbons concentration (361.13 mug/L) is estimated to be degraded completely in 6 years under the present hydrogeological and environmental conditions.