Methods, apparatus, and systems for low-flow sampling of a fluid source. One illustrative embodiment of the invention is directed to a method for manually sampling a fluid source in a well, including exerting a downward pressure on a sample tube, disposed within a well insert, to allow fluid to pass from the well through a portion of the well insert and into the sample tube to fill at least a portion of the sample tube with the fluid. Another illustrative embodiment of the invention is directed to a method for securing a well insert to an inner wall of a well, including arranging stabilizers on the well insert in a non-extended position, introducing the well insert into the well, arranging the stabilizers on the well insert in an extended position, and expanding a portion of a gasket on the well insert such that the portion of the gasket conforms to a circumference of the inner wall of the well.

CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-Part of application Ser. No. 09/883,068, filed Jun. 15, 2001, now abandoned, which application is hereby incorporated herein by reference and claims the benefit of Provisional Application No. 60/211,947, filed Jun. 16, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods, apparatus, and systems for low-flow sampling of a fluid source. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a well and groundwater sampling. Wells are used in the environmental and water supply industries, among other things, to collect samples of groundwater for chemical analysis. A typical well, shown in FIG. 1, is comprised of a slotted section of pipe  37  (the “well screen”) located at the bottom of the well, a well point  39  which plugs the bottom of well screen  37  and sections of solid pipe  41  (the “riser pipe”) which thread onto well screen  37  and each other to bring the well to a ground surface  49 . The slots in well screen  37  are narrow enough (e.g., on the order of hundredths of an inch) to keep out soil particles, but allow in groundwater. The water level within a well having a portion of well screen  37  above a water table  43  (e.g., water table well  45 ) is the same as the level of water table  43  since atmospheric pressure alone is acting equally on both. If the entire well screen is located beneath water table  43  (e.g., in deep well  47 ), additional pressures, such as the weight of the water above, act on the groundwater. The pressures acting on a unit measure of groundwater are referred to as “head pressure.” Groundwater will flow from areas of high head pressure to areas of low head pressure. Groundwater entering deep well  47  will usually flow upward into riser pipe  41  until it reaches equilibrium. The water level at equilibrium is sometimes referred to as the piezometric surface. 
     Since the water in a water table well is exposed to the atmosphere, it is not considered representative of groundwater outside the well screen. In a deep well, as a result of the water flowing into riser pipe  41  until equilibrium is reached, standing water may be present in the riser pipe between groundwater sampling events. This water may be present for weeks, months, or longer, and is also not considered representative of the groundwater outside of the well screen. In conventional systems, up to six volumes of standing water must be purged from the well. The purged water typically needs to be treated to remove contaminants before it can be discharged, which may require costly and time-intensive off-site treatment. 
     SUMMARY OF THE INVENTION 
     One illustrative embodiment of the invention is directed to a method for manually sampling a fluid source in a well, comprising an act of exerting a downward pressure on a sample tube, disposed within a well insert, to allow fluid to pass from the well through a portion of the well insert and into the sample tube to fill at least a portion of the sample tube with the fluid. 
     Another illustrative embodiment of the invention is directed to a method for securing a well insert to an inner wall of a well, comprising acts of arranging stabilizers on the well insert in a non-extended position; introducing the well insert into the well; arranging the stabilizers on the well insert in an extended position; and expanding a portion of a gasket on the well insert such that the portion of the gasket conforms to a circumference of the inner wall of the well. 
     A further illustrative embodiment of the invention is directed to an apparatus, comprising a well insert including a piston, the well insert being mounted to an inner surface of a well; and a sample tube, with means for activating the piston, disposed within the well insert. 
     Another illustrative embodiment of the invention is directed to an apparatus, comprising a well insert for insertion into a well; stabilizers mounted to the well insert, the stabilizers being movable from a non-extended position to an extended position such that when the stabilizers are in an extended position, the stabilizers extend to a location at or beyond a diameter of an inner wall of the well; and a gasket mounted to the well insert, the gasket being radially extendable from a non-extended position to an extended position such that when the gasket is in an extended position, the gasket conforms to a circumference of the inner wall of the well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings are not intended to be drawn to scale. In the drawings, like elements have been given like reference characters. 
     FIG. 1 is a cross-sectional view of a typical water table well and deep well. 
     FIG. 2 is a cross-sectional view of a well insert according to an embodiment of the invention, shown in a water table well and deep well. 
     FIG. 3 is a cross-sectional view of component parts of the well insert of FIG.  2 . 
     FIG. 4 is a cross-sectional view of component parts of the lower portion of the well insert of FIG.  2 . 
     FIG. 5 is a cross-sectional view of component parts of the upper portion of the well insert of FIG.  2 . 
     FIG. 6 shows a cross-sectional side view and top view of the stabilizer, gasket, and upper portion of the well insert of FIG.  2 . 
     FIG. 7 is a cross-sectional view of the piston of the well insert of FIG. 2 and a sample tube. 
    
    
     DETAILED DESCRIPTION 
     One aspect of the present invention is directed to a method and apparatus for controlling the flow of groundwater into a deep well. Another aspect of the invention is directed to a method and apparatus for low-flow groundwater sampling within a water table or a deep well. Although these two aspects of the present invention are advantageously employed together in accordance with various illustrated embodiments of the invention, the present invention is not limited in this respect, as each of these aspects of the present invention can be employed separately. 
     One illustrative embodiment of an apparatus for controlling the flow of groundwater into a well and sampling the groundwater within the well at a low flow rate is shown in FIGS. 2-7. FIG. 2 illustrates, from left to right: (A) a well insert  23 , in a shallow well  33 , having a piston  25  in an closed position; (B) well insert  23 , in shallow well  33 , having piston  25  in an open position; and (C) well insert  23 , in a deep well  35 , having piston  25  in a closed position. In the shallow well, a well screen  37  traverses water table  43 , while gasket  11  is above water level  43 . In the deep well, both well screen  37  and gasket  11  are submerged below water table  43 . Aspects of the present invention are suited for use with both shallow and deep wells. 
     FIG. 3 illustrates an exploded view of well insert  23 , which includes a lower portion  51  comprising a piston  25  and a piston housing  27 ; a middle portion  29  comprising a short length of pipe threaded on each end; and an upper portion  53  comprising gasket  11  and a housing assembly that includes an upper housing portion  31  and a lower housing portion  49 . The lower portion  51  and upper portion  53  thread onto the middle portion  29  to produce well insert  23 . O-rings (not shown) may be used at the interface between connecting elements to ensure a water-tight seal. 
     FIG. 4 illustrates an exploded view of the lower portion  51  of well insert  23 . Lower portion  51  of well insert  23  is assembled by inserting a compression spring  1  into a groove  2  on the underside of a top  3  of piston  25 , inserting compression spring  1  and top  3  of piston  25  into an opening  4  within a top  7  of piston housing  27 , inserting a bottom  5  of piston  25  into an opening  6  within a conical section  22  of piston housing  27 , and threading top  3  and bottom  5  of piston  25  together within piston housing  27 . The lower portion  51  is then threaded onto middle portion  29 , which is in turn attached to upper portion  53  by threading a bottom  8  of lower housing portion  49  (FIG. 5) onto middle portion  29 , producing the well insert. 
     FIG. 5 illustrates an exploded view of the upper portion  53  of well insert  23 , which includes upper housing portion  31  and lower housing portion  49 , each threaded to allow for connection to the other. In particular, a bottom  12  of upper housing portion  31  is threaded to receive a top  10  of lower housing portion  49 . As discussed above, when these are coupled together, they create a recess (i.e., gasket chamber  59 ) to house gasket  11  and an expander  55 , which may be ring-shaped and be tapered. Tightening upper housing portion  31  and lower housing portion  49  reduces the distance between the housings. The reduction in size of the gasket chamber forces expander  55  between gasket  11  and a wall of gasket chamber  59 , which compresses gasket  11  in the vertical, as shown by arrows A, and expands gasket  11  outward in the radial, as shown by arrows B. If well insert  23  is inserted into a well, the expanded gasket  11  will press against both the upper portion and the inner wall of the well. This pressure can be sufficient to secure the well insert within the well. 
     In addition to securing well insert  23  to the inner wall of the well, gasket  11  creates a seal that is substantially or fully impermeable to air and/or fluids (e.g., water) when in the expanded position. In a water table well, this seal isolates the groundwater from the air in the riser pipe, thereby substantially reducing the amount of volatilization that may occur within the well. In a deep well, the seal provides two advantages: 1) groundwater flowing into the screen is prevented from mixing with the “stagnant” water in the riser pipe; and 2) since the water in the riser pipe above the gasket can be removed, there is no cross-contamination of sampling equipment lowered into the well. In both the water table well and deep well examples, sealing the well at the top of the well screen results in less alteration of chemicals or chemical concentrations in groundwater as it flows through the well screen. Therefore, chemical analysis of groundwater in the well screen can be considered “representative” of the chemical makeup of groundwater in the aquifer. 
     An additional advantage provided by the air-tight and water-tight seal formed by gasket  11  is the prevention of contamination of a well by outside sources. Wells are susceptible to chemicals and other contaminants that may be introduced into the well, either by an environmental contamination or a deliberate act. These contaminants may infiltrate the well and surrounding ground, and thereby taint a large region of groundwater. Because the gasket described above is located below the surface of the ground, it is inaccessible to removal without proper equipment. Thus, the gasket serves to safeguard the well from accidental corruption or vandalism. 
     FIG. 6 shows from top to bottom: (A 1 ) a top view of upper housing portion  31 , (A 2 ) a cross-sectional side view of upper housing portion  31 , (B 1 ) a top view of gasket  11 , (B 2 ) a cross-sectional side view of gasket  11 , (C) and a cross-sectional top view of lower housing portion  49  with stabilizers  9 . According to one embodiment of the invention, stabilizers  9  may be used to secure the well insert to the inner wall of a well before expansion of gasket  11 . Stabilizers  9  may be recessed into lower housing portion  49  and may pivot on stainless steel pins  13 . Stabilizers  9  may be made from stainless steel and may be wedge-shaped. The diameter of lower housing portion  49  across stabilizers  9 , when the stabilizers  9  are pivoted (e.g., counterclockwise), is slightly larger than the inner diameter of the well. When the well insert is situated within the well, clockwise rotation causes stabilizers  9  to “catch” on the sides of the well, preventing the lower housing portion  49  from turning. This allows upper housing portion  31  to tighten, forcing expander  55  (FIG. 5) into an opening  61  of gasket  11 , and in turn forcing the gasket to expand radially in the direction of arrows B against the inner wall of the well to secure the well insert in place. The well insert may be installed in a well by lowering it downward on extension rods (not shown) until the gasket is located just above the well screen. A turning tool (not shown) connected to the end of an extension rod may fit into grooves  14  cut into the top of upper housing portion  31 . The turning tool may be locked into grooves  14  by a spring attached to the bottom of the tool. 
     FIG. 7 illustrates a sample tube  15  having flow-control point  16 . To collect a groundwater sample, sample tube  15  is lowered into the well until flow-control point  16  is seated in a conical point chamber  17  located at the top of piston  25 . Downward pressure on the sample tube overcomes compression spring  1  (FIG.  4 ), allowing the piston to extend into the groundwater and expose holes  18  in the shaft of piston  25 , which are otherwise occluded by piston housing  27  (FIG.  4 ). The head pressure at the holes forces groundwater to flow into holes  18  and through a hollow center  19  of piston  25 , through a hole  20  in flow-control point  16 , and into sample tube  15 . A free floating check ball  57  may be provided, which is displaced from hole  20  when fluid flows into sample tube  15 . Check ball  57  returns to its original position covering hole  20  when sample tube  15  is removed from the well, and thereby prevents fluid from escaping sample tube  15  via hole  20 . The rate of flow of groundwater into the sample tube is controlled by the size of hole  20 . A rate of flow that is close to the natural rate of flow of water entering the well is desirable, as a low rate of flow is less likely to disturb sediment that may contaminate the groundwater in sample tube  15 . Groundwater continues to flow into sample tube  15  until the downward pressure is released, causing compression spring  1  to retract piston  25  into the piston housing by seating conical section  21  of piston  25  against a conical section  22  of piston housing  27  (FIG.  4 ). The retraction of piston  25  within the piston housing creates a water-tight seal by occluding holes  18 . Water may alternatively cease to flow into sample tube  15  when the water level in sample tube  15  equilibrates. Once a sample has been obtained, sample tube  15  can be removed from the well. 
     Above-described aspects of the present invention are advantageous because they may: 1) isolate the water in the well screen from either the air in the riser pipe in a water table well or the stagnant water in the riser pipe in a deep well, 2) control the flow of groundwater into a deep well, and 3) allow collection of a groundwater sample at a very low rate of flow. The first advantage has been previously discussed. The second and third relate to purging the well. Conventional sampling protocol calls for removing up to 6 well volumes prior to collecting a sample. This procedure is referred to as purging the well, and the purpose is to remove any chemically altered water in the well, bringing water from the aquifer into the well. Purging adds a significant expense to sample collection since it is time consuming, and purge water needs to be collected and treated to remove any contaminants before it can be discharged back to the environment. The United States Environmental Protection Agency (the “USEPA”) has recently documented collecting groundwater at low-flow rates (0.1 to 0.5 milliliters per minute, “mL/min”) to avoid disturbing the water column, which can introduce particulate matter or colloids into the well. These can attract contaminants that would be included in analysis of the water resulting in an unrepresentative measurement of contaminant concentrations in the sample. Dedicated sample apparatus and low-flow sampling protocol can minimize the disturbance of the water column during sampling. Part and parcel with low-flow sampling is the concept of “no-purge”. This states that groundwater flowing through the well screen can be considered representative if the water column is not disturbed during sample collection, thus eliminating the need to purge the well prior to collecting a sample. The present invention allows both low-flow and no-purge sampling capability. The rate of flow into the sample tube is controlled by the diameter of the hole through the flow-control point with diameters less than ⅛ inch producing flow rates less then 1 mL/min. Purging is not required in a well having a well insert according to the present invention, since the well insert can be permanently installed in the well, thereby eliminating any disturbances to the water column which can result from the installation of a sampling system (e.g., a submersible pump or bailer) at the time of sampling, and since the gasket of the well insert seals the well screen from air or water in the riser pipe. 
     The present invention may be implemented without pumps, which advantageously reduces the cost of the sampling apparatus. Further, the well insert may be retained in the well between sampling events, which eliminates the need of constructing and deconstructing the sampling apparatus between events. Retaining the well insert between sampling events also allows the gasket to protect the well from exposure to air and potential contamination between sampling. 
     It should be appreciated that although the invention has been described in the context of sampling groundwater, other fluids may alternatively be sampled according to the invention. It should further be appreciated that the materials noted for use in the apparatus described are given for example only. The well insert and sample tube may be made from a number of plastics, metals, and other materials that are relatively impermeable and unreactive to water. Similarly, the gasket may be made from plastic, rubber, and other materials that are relatively impermeable and unreactive to water, and may have a cylindrical shape or disk shape, for example. 
     Having thus described several illustrative embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.