and migrate in the aquifer to contaminate groundwater. In the drilling process, the
mud pit may cut the unconfined aquifer, typically in areas with shallow aquifers. In
this case, without seepage control measures, leakage will be inevitable, and lead to
groundwater contamination with the direct entry of pollutants into the aquifers (Xu
and Chen 2017).
Groundwater pollution sources include drilling wastewater, waste mud, oil
sludge, oily sewage, and injection water (Zhang et al. 2010). Under normal circumstances, these sources do not have significant influence on groundwater as long
as they are timely collected, prevented from seepage and landfilled. However,
groundwater quality may be undermined in the event of overflow in the rainy
season, collection failure and leakage.
(1) Types of pollutants
Pollutants mainly fall into two categories: VOCs and SVOCs. Among them, VOCs
are mainly composed of olefins, alkanes and benzene, and SVOCs composed of
PAHs.
(2) Direction of groundwater flow fields
Soil pollution in OCS sites is obviously characterized by blocks. It is prominent
near oil tanks, oil reservoirs and oil drains, with high detection rate of organic
pollutants. Groundwater contamination is not limited to blocks as flowing shallow
groundwater in the whole oil processing and production area is subject to contiguous contamination. Therefore, the direction of groundwater flow fields is of
great significance to the spread of contamination.
(3) Soil lithology
In the depth range of pollution, pollutants are more concentrated in cohesive soil
than in non-cohesive soil; pollutant concentration is relatively high in groundwater
fluctuation zone; and pollutants in dry sand layer mainly exist in the gas phase with
high concentration. During the infiltration process, pollutants are redistributed
under the influence of lithology and its characteristics (water content, organic matter
content, and etc.). The organic matter content of clay stratum is higher than that of
silt stratum because cohesive soil adsorbs greater organic pollutants than silt
according to the theory of similar polarity.
(4) Depth to water table
According to the results of investigation and analysis, in most areas of contaminated
sites, as the depth to water table increases, the concentrations of halogenated
hydrocarbons, monocyclic aromatic hydrocarbons and chlorinated benzenes
decrease while the dissolved oxygen concentration (DOC), ORP and pH show an
upward trend. In other words, shallow groundwater has low DOC, ORP and pH,
implying a high degree of contamination, and deep groundwater has high DOC,
ORP and pH, implying a low level of contamination.
3.1 Indicators of Pollution Source Intensity Evaluation
37
mud pit may cut the unconfined aquifer, typically in areas with shallow aquifers. In
this case, without seepage control measures, leakage will be inevitable, and lead to
groundwater contamination with the direct entry of pollutants into the aquifers (Xu
and Chen 2017).
Groundwater pollution sources include drilling wastewater, waste mud, oil
sludge, oily sewage, and injection water (Zhang et al. 2010). Under normal circumstances, these sources do not have significant influence on groundwater as long
as they are timely collected, prevented from seepage and landfilled. However,
groundwater quality may be undermined in the event of overflow in the rainy
season, collection failure and leakage.
(1) Types of pollutants
Pollutants mainly fall into two categories: VOCs and SVOCs. Among them, VOCs
are mainly composed of olefins, alkanes and benzene, and SVOCs composed of
PAHs.
(2) Direction of groundwater flow fields
Soil pollution in OCS sites is obviously characterized by blocks. It is prominent
near oil tanks, oil reservoirs and oil drains, with high detection rate of organic
pollutants. Groundwater contamination is not limited to blocks as flowing shallow
groundwater in the whole oil processing and production area is subject to contiguous contamination. Therefore, the direction of groundwater flow fields is of
great significance to the spread of contamination.
(3) Soil lithology
In the depth range of pollution, pollutants are more concentrated in cohesive soil
than in non-cohesive soil; pollutant concentration is relatively high in groundwater
fluctuation zone; and pollutants in dry sand layer mainly exist in the gas phase with
high concentration. During the infiltration process, pollutants are redistributed
under the influence of lithology and its characteristics (water content, organic matter
content, and etc.). The organic matter content of clay stratum is higher than that of
silt stratum because cohesive soil adsorbs greater organic pollutants than silt
according to the theory of similar polarity.
(4) Depth to water table
According to the results of investigation and analysis, in most areas of contaminated
sites, as the depth to water table increases, the concentrations of halogenated
hydrocarbons, monocyclic aromatic hydrocarbons and chlorinated benzenes
decrease while the dissolved oxygen concentration (DOC), ORP and pH show an
upward trend. In other words, shallow groundwater has low DOC, ORP and pH,
implying a high degree of contamination, and deep groundwater has high DOC,
ORP and pH, implying a low level of contamination.
3.1 Indicators of Pollution Source Intensity Evaluation
37