Water stored in the soil and rock voids (pores, fissures, dissolution gaps) is collectively referred to as groundwater. The groundwater is buried in different depths of the stratum. The changes of the fluidity and water quality parameters are slow compared with the surface water.
(1) Layout principle of monitoring network
(1) In general and macro, different hydrogeological units should be controlled, and the environmental quality and spatial changes of groundwater quality of underground water system in the region should be reflected;
(2) The monitoring focuses on aquifers for water supply purposes;
(3) Monitor the key polluted areas and areas where pollution may occur, monitor the pollution degree and dynamic changes of the pollution sources to the groundwater, so as to reflect the pollution characteristics of the groundwater in the area;
(4) It can reflect the water supply source of groundwater and the hydraulic connection between groundwater and surface water; Monitor the funnel area, ground subsidence and special hydrogeological problems of groundwater level drop;
(5) Considering the influence of industrial construction projects, mine development, water conservancy projects, oil development and agricultural activities on groundwater;
(6) The principle of monitoring the density of network layout is that the main water supply area is dense and the general area is sparse; The urban area is dense and the countryside is sparse; Groundwater pollution
The serious areas are dense and the non pollution areas are sparse. Obtain enough representative environmental information with the minimum monitoring points as possible; The monitoring points shall be selected from the frequently used civilian wells, production wells and spring water as far as possible.
（2） Investigation, research and data collection
Before setting up monitoring point network, relevant local hydrological and geological data shall be collected, including:
(1) Geological map, profile, relevant parameters of existing well (well location, drilling date, well depth, well forming method, aquifer location, pumping test data, drilling unit, use value, water quality data, etc.);
（2） The geographical distribution and hydrological characteristics of the river, river, lake and sea as the local groundwater supply source (water level, water depth, velocity and flow), water conservancy facilities, utilization of surface water and water quality;
(3) Aquifer distribution, groundwater supply, runoff and discharge direction, groundwater quality type and groundwater resources development and utilization; Spring water exposure, origin type, supply source, flow rate, water temperature, water quality and utilization;
(4) Regional planning and development, distribution of urban and industrial areas, resource development and land use, application of fertilizer and pesticide, water pollution sources and sewage discharge characteristics.
(3) Sample point settings
Because of the complexity and particularity of hydrogeology and other factors, the setting of groundwater sampling points is more complex. The density of monitoring points in general national controlled groundwater is not less than 100km2 0.1 well, each county shall have at least 1-2 wells, and the plain (including basin) area is generally every 100km2 0.2 well, important water source or polluted areas shall be properly encrypted. Desert area, hill area and karst mountain area can be selected as required to set monitoring points in typical representative areas. The density of monitoring points of provincial and municipal groundwater monitoring points can be determined according to the specific situation and relevant specifications.
Monitoring points (monitoring wells) should be set in the areas where groundwater is the main water source, the area with high incidence of drinking water type local diseases (such as high fluorine disease), and the areas with great influence on the regional groundwater, such as sewage irrigation area, garbage disposal area, groundwater return irrigation area and large mine drainage area.
In order to understand the types, distribution and diffusion conditions of pollutants, and to understand the stratification and flow direction of groundwater, background value monitoring wells and pollution control monitoring wells are usually needed.
one The layout of monitoring wells (points) with background value
According to the regional hydrogeological unit and the main groundwater supply source, one or more background value monitoring wells are set up upstream of groundwater flow around the polluted area. The background value monitoring well should be far away from the urban residential area, industrial area, pesticide fertilizer application area, agricultural irrigation area and traffic main road as far as possible.
two The layout of pollution control monitoring well (point)
The distribution of pollution sources and the diffusion of pollutants in underground water are the primary factors to be considered in the layout of pollution control monitoring wells. According to the local groundwater flow direction, distribution of pollution sources and the form of pollutant diffusion in underground water, pollution control monitoring wells can be arranged by combining point to surface, with the monitoring focus on the water supply water source protection area.
① Pollutants in the seepage pit, seepage well and solid waste stacking area spread in the area with high permeability of aquifer. The monitoring wells should be arranged along the direction of groundwater, and controlled by parallel and vertical monitoring lines; Pollutants in the seepage pit, seepage well and solid waste stacking area spread in the area with small permeability of aquifer, and monitoring lines can be arranged in cross shape near the pollution source for control;
② When industrial wastewater, domestic sewage and other pollutants are discharged or leaked along the river channel to spread with belt pollution, the monitoring line perpendicular to the river channel shall be set by grid distribution method according to the state of the river channel, the flow direction of groundwater and the geological conditions; The sewage in sewage irrigation area and residential area without sanitation facilities is easy to cause massive pollution to the surrounding environment in a large area, and monitoring points shall be arranged in a parallel and vertical way to the direction of groundwater;
③ The funnel area where the groundwater level drops mainly forms the lateral pollution diffusion near the mining funnel, monitoring and measuring points shall be set in the funnel center. If necessary, monitoring lines can be arranged outside according to the cross shaped or radial shape through the funnel center;
④ The pollution range of the strong diffusion area or old pollution source with good permeability may be large, and the monitoring line can be extended appropriately. Otherwise, it can only be distributed near the pollution source.
（4） Determination of sampling time and frequency
(1) The background value monitoring well and the regional controlled pore pressure well are sampled once a year in the dry season;
(2) The pollution control monitoring well is sampled once a month and six times in the whole year; If a monitoring project is lower than one fifth of the control standard value for two consecutive years, and there is no new pollution source near the monitoring well, and the existing pollution source discharge is not increased, the project can be sampled once a year in the dry water period. Once the monitoring results are greater than one fifth of the control standard value, or there are new pollution sources near the monitoring wells or the new discharge quantity of existing pollution sources, the normal sampling frequency will be restored;
(3) The groundwater monitoring logging, which is used as the centralized water supply for drinking water, is sampled once a month;
(4) The sampling time of monitoring wells in the same hydrogeological unit should be concentrated as far as possible, and the date span should not be too large;
(5) In case of special circumstances or pollution accidents, the sampling frequency shall be increased at any time when it may affect the quality of groundwater.