ARCHIVED REPORTS_XR0002852 - Laserfiche WebLink

Home Browse Search

1 flow purging has the advantage of minimizing mixing between tion-reduction potential,temperature and turbidity should be the overlying stagnant casing water and water within the used to determine when formation water is accessed during screened interval. purging. In general,the order of stabilization is pH,tempera- ture,and specific conductance,followed by oxidation- A. Low-Flow Purging and Sampling reduction potential, dissolved oxygen and turbidity. Tempera- ture and pH,while commonly used as purging indicators, are Low-flow refers to the velocity with which water actually quite insensitive in distinguishing between formation enters the pump intake and that is imparted to the formation water and stagnant casing water; nevertheless,these are pore water in the immediate vicinity of the well screen. It important parameters for data interpretation purposes and does not necessarily refer to the flow rate of water discharged should also be measured. Performance criteria for determi- j at the surface which can be affected by flow regulators or nation of stabilization should be based on water-level draw- restrictions. Water level drawdown provides the best indica- down, pumping rate and equipment specifications for measur- tion of the stress imparted by a given flow-rate for a given ing indicator parameters. Instruments are available which hydrological situation. The objective is to pump in a manner utilize in-line flow cells to continuously measure the above that minimizes stress(drawdown)to the system to the extent parameters. practical taking into account established site sampling objectives. Typically,flow rates on the order of 0.1 -0.5 L/min It is important to establish specific well stabilization ' are used, however this is dependent on site-specific criteria and then consistently follow the same methods hydrogeology. Some extremely coarse-textured formations thereafter, particularly with respect to drawdown,flow rate have been successfully sampled in this manner at flow rates and sampling device. Generally,the time or purge volume to 1 Vmin. The effectiveness of using low-flow purging is required for parameter stabilization is independent of well intimately linked with proper screen location, screen length, depth or well volumes. Dependent variables are well diam- and well construction and development techniques. The eter,sampling device, hydrogeochemistry, pump flow rate, reestablishment of natural flow paths in both the vertical and and whether the devices are used in a portable or dedicated horizontal directions is important for correct interpretation of manner. If the sampling device is already in place(i.e., the data. For high resolution sampling needs,screens less dedicated sampling systems),then the time and purge than 1 m should be used. Most of the need for purging has volume needed for stabilization is much shorter. Other been found to be due to passing the sampling device through advantages of dedicated equipment include less purge water the overlying casing water which causes mixing of these for waste disposal,much less decontamination of equipment, stagnant waters and the dynamic waters within the screened less time spent in preparation of sampling as well as time in interval. Additionally,there is disturbance to suspended the field,and more consistency in the sampling approach sediment collected in the bottom of the casing and the which probably will translate into less variability in sampling displacement of water out into the formation immediately results. The use of dedicated equipment is strongly recom- adjacent to the well screen. These disturbances and impacts mended at wells which will undergo routine sampling over can be avoided using dedicated sampling equipment,which time. precludes the need to insert the sampling device prior to ' purging and sampling. If parameter stabilization criteria are too stringent, then minor oscillations in indicator parameters may cause isolation of the screened interval water from the purging operations to become unnecessarily protracted. It overlying stagnant casing water may be accomplished using should also be noted that turbidity is a very conservative low-flow minimal drawdown techniques. If the pump intake is parameter in terms of stabilization. Turbidity is always the located within the screened interval, most of the water last parameter to stabilize. Excessive purge times are pumped will be drawn in directly from the formation with little invariably related to the establishment of too stringent turbidity mixing of casing water or disturbance to the sampling zone. stabilization criteria. It should be noted that natural turbidity However,if the wells are not constructed and developed levels in ground water may exceed 10 nephelometric turbidity properly,zones other than those intended may be sampled. units(NTU). At some sites where geologic heterogeneities are sufficiently different within the screened interval, higher conductivity C.Advantages and Disadvantages of Low-Flow zones may be preferentially sampled.This is another reason (Minimum Drawdown)Purging to use shorter screened intervals, especially where high spatial resolution is a sampling objective. In general,the advantages of low-flow purging include: B. Water Quality Indicator Parameters • samples which are representative of the mobile load of contaminants present(dissolved and colloid-associ- It is recommended that water quality indicator ated); parameters be used to determine purging needs prior to - minimal disturbance of the sampling point thereby sample collection in each well. Stabilization of parameters minimizing sampling artifacts; such as pH,specific conductance, dissolved oxygen,oxide- - less operator variability, greater operator control; 5