Abstract:
An ultrasonic flow meter has been adapted for such measurements in the submarine environment. Connected to a collection funnel, the meter houses two piezoelectric transducers mounted at opposite ends of a cylindrical flow tube. By monitoring the perturbations of fluid flow on the propagation of sound waves inside the flow tube, the ultrasonic meter can measure both forward and reverse fluid flows in real time. Laboratory and field calibrations show that the ultrasonic meter can resolve groundwater discharges in both the forward and reverse directions on the order of 0.1 μm/s (&lt;1 cm/d), and it is sufficiently robust for deployment in the field for several days. Data collected with the meter elucidate the temporal and spatial heterogeneity of submarine groundwater discharge and its interplay with tidal loading and other driving forces. A negative correlation between the discharge and tidal elevation can be observed.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an apparatus and method for measuring the velocity and other characteristics of submarine groundwater discharge as it enters or exits a surface water body, and more particularly to a remotely deployable time transient seepage meter that utilizes ultrasonic technology to continuously quantify and record the measured information. The present invention is intended for use in environmental fields including hydrology, oceanography, geology, ecology and all other related fields.  
       BACKGROUND OF THE INVENTION  
       [0002]     Understanding the exchange between seepage and overlying surface water has become increasingly important due to the potential impacts to the environment resulting from anthropogenic land uses. The key input for submarine groundwater discharge (SGD) in near-shore environments is believed to be the discharge from land to surface water induced by the hydraulic gradient in the terrestrial aquifer. However, significant contribution to SGD may also derive from groundwater circulation and oscillating flow induced by tidal stage as well as salinity and thermal variations. This discharge carries with it contaminants and/or nutrients, dissolved and/or colloidal, that have the potential to impact the chemical budget of surface water ecosystems. This impact, along with other biological and physical impacts, may be heightened in smaller bodies of water such as embayments or lagoons due to their limited volume and restricted fluid exchange with the open ocean.  
         [0003]     A major obstacle in studying SGD is accurately measuring groundwater seepage across the sediment-water interface. Discharge rates may be as low as &lt;1 cm/day and these low rates make quantification of SGD inherently difficult. In addition, the ebullition of gas from sediments is a common event, further increasing the difficulty of accurately measuring SGD.  
         [0004]     Current methodologies for measuring SGD have included a system that utilizes a 4-liter plastic collection bag and a cut off section of a 55 gallon drum as described by D. R. Lee, in “A Device for Measuring Seepage Flux in Lakes and Estuaries,”  Limnology and Oceanography,  22: 140-147, 1977. Using this device, the open-ended section of a cut off section of a 55-gallon drum is inserted into the sediment. Attached to the drum via an outflow port is a 4-liter plastic bag that collects the seepage. The volume of the bag and sampling interval are recorded and the specific discharge velocity is obtained by dividing the volume of collected seepage over the time interval by the area of the drum. Although this method can be effective, various errors have been associated with the device that must be corrected for prior to sampling. Another disadvantage to this method is that it is quite labor intensive since the plastic bags need to be monitored and replaced continuously. In addition, data collected are averages over the specified time interval and may not fully quantify short term events. Furthermore, this method is incapable o measuring reverse flow.  
         [0005]     Continuous logging seepage meters have been developed utilizing heat-pulse technology as described by M. Taniguchi and Y. Fukuo, in “Continuous Measurements of Ground-Water Seepage Using an Automatic Seepage Meter,”  Ground Water,  31, no. 4: 675-679, 1993. This method, however, cannot be monitored during deployment and may therefore malfunction during the collection period without notice. Another disadvantage of this method is that it cannot measure seepage in intertidal environments in which the seepage meter becomes periodically air bound during low tide events. In addition, variations in the water density and temperature can also affect the accuracy of the heat pulse method.  
         [0006]     Piezometric head measurements have also be used to estimate the specific discharge of groundwater to surface waters. This method requires the installation of monitoring wells offshore to monitor the hydraulic head beneath the surface water. The method can determine if water is entering or exiting the surface water but in order to determine the specific discharge estimates of the hydraulic conductivity of the sediment are needed. However, this method is not a direct measurement of seepage but and estimate based on head measurements and sediment conductivity.  
         [0007]     Accordingly, there is a need for a remotely-deployable device capable of accurately measuring SGD in both the forward and reverse flow directions.  
       SUMMARY OF THE INVENTION  
       [0008]     It is the object of the present invention to overcome the shortcomings of previous methods to quantify SGD. The invention is intended to provide a robust, time transient meter that is able to continuously record the wide range of seepage rates observed in the field while providing a less labor intensive and more accurate method of quantifying SGD in both the forward and reverse flow directions.  
         [0009]     One advantage of the present invention is to be able to continuously record groundwater discharge in subsurface (intertidal and subsurface) environments. Another feature of the present invention is the ability to utilize velocity of sound data collected by the meter to determine The salinity and therefore source of the water passing through the seepage meter. This feature provides an advantage over other methods by differentiating terrestrial groundwater from surrounding surface water in marine environments. Therefore, the source of the discharge and any toxic or hazardous constituents present in the discharge is more easily determined and quantified. Another advantage of the present invention is the ability to accurately and continuously quantify very low flow rates regardless of changes in the temperature, density or the affect of ebullition of gases from the sediments.  
         [0010]     The above and other objects, advantages, and features are accomplished by providing a mechanism capable of quantifying groundwater seepage rate and quality. A seepage meter in accordance with the present invention generally comprises a funneling device to collect the groundwater flow through an inlet which then discharges through this device via a discharge outlet, for recording water temperature, tidal stage, and electrical conductance using probes mounted in the device; an ultrasonic flow tube, for accepting the discharge from the collection device through a collection inlet and discharging the groundwater through a discharge outlet; a data logger, connected to the flow tube which sends signal to the flow tube and receives data from the flow tube which then is used to determine the flow rate of groundwater through the flow tube.  
         [0011]     Further, in accordance with the above objects, advantages, and features the present invention provides a device that quantifies the rate of submarine groundwater discharge using inlet means, outlet means, connected to the inlet and outlet means for determining seepage rate; control means, for sending signal to the flow tube; recording means, to quantify seepage rate using travel time data from the flow tube; valve means, for transferring discharge from the collection device to an additional discharge outlet connected to a pump for sampling.  
         [0012]     In further accordance with the above objects, advantages, and features the present invention provides a methodology for quantifying submarine groundwater discharge by utilizing a) collecting water through a collection funnel, b) directing the collected water through an in line ultrasonic flow tube, c) using a data logger to quantify and record flow rate, and d) repeating steps a) through c). 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS  
       [0013]      FIG. 1  is a perspective view of a preferred embodiment of an ultrasonic seepage meter in accordance with the present invention.  
         [0014]      FIG. 2  is a partial view of the ultrasonic seepage meter of  FIG. 1 .  
         [0015]      FIG. 3  is a partial cutaway view of the flow meter of the ultrasonic seepage meter of  FIG. 1 .  
         [0016]      FIG. 4  is a partial side cutaway view of the ultrasonic seepage meter of  FIG. 1 .  
         [0017]      FIG. 5  is a graph illustrating data recorded using the ultrasonic seepage meter of  FIG. 1  in West Neck Bay. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0018]      FIGS. 1 and 2  illustrate a preferred embodiment of an ultrasonic seepage meter  100  in accordance with the present invention. Generally, ultrasonic seepage meter  100  comprises collection funnel  110 , flow meter  140 , and controller logger  160 . Collection funnel  110 , as shown in  FIGS. 1 and 2 , further comprises an open bottom  112  and an enclosed top  114 . The sides of collection funnel  110  form a square, each side being 0.46 m in length (corresponding to a capture area A=0.21 m 2 ), and at least 10 cm tall. However, it can be appreciated that collection funnel  110  may be of many different types of geometries and still be within the spirit of the present invention. Extending from side  116  of funnel  110  is discharge outlet  120 . Discharge outlet  120  may also further comprise valve  122 , which may be a ball valve. In a preferred embodiment, the top  114  of funnel  110  is angled so that the side  116  with discharge outlet  120  is slightly higher than the other side of funnel  110  (see  FIG. 4 ).  
         [0019]     Flow meter  140  is fluidly connected to discharge outlet  120 . In a preferred embodiment, tygon tubing  124  is used to fluidly connect discharge outlet  120  with flow meter  140 .  
         [0020]     As shown in  FIG. 3 , flow meter  140  comprises inlet  144 , outlet  146 , ultrasonic flow tube  150 , and piezoelectric transducers  152 . In a preferred embodiment, ultrasonic flow tube  150  is a Controlotron ultrasonic flow tube (U.S. Pat. No. 5,467,321). Inlet  144 , outlet  146 , and ultrasonic flow tube  150  are fluidly connected to each other by tygon tubing  154 . As water discharges from the collection funnel  110  it enters the flow meter  140  at inlet  144  and passes through tygon tubing  152  into ultrasonic flow tube  150 . Two piezoelectric transducers  152  mounted at opposite ends of ultrasonic flow tube  150 . Outlet  146  may further comprise a valve  142 , which may be a ball valve. The components of the flow meter  140  are enclosed within watertight casing  159 .  
         [0021]     Piezoelectric transducers  152  continually generate bursts of ultrasonic signals from one end of ultrasonic flow tube  150  to the other end. Typically ˜250 bursts are transmitted per second, and each burst is made up of ˜40 periodic ultrasonic waves. As water flows through ultrasonic flow tube  150 , the speed by which the water is moving through the flow tube affects the speed of the ultrasonic signals. Arrival of the ultrasonic signals is continuously monitored by piezoelectric transducers  152  which are in communication with control logger  160 . Measurement of the sound waves provides the velocity data needed. The discharge exits through outlet  146 .  
         [0022]     Controller logger  160  is attached to the outside of the funnel  110  and is encased in water tight housing  161 . Control logger  160  comprises a control module and a data logger and is in operative communication with flow meter  140 . Control logger  160  controls the operation of flow meter  140  and also collects data received from flow meter  140 . In a preferred embodiment (not shown), housing  161  has a clear end cap which displays controls that can be changed by the use of a magnetic wand. This allows for controller logger  160  to be programmed while underwater. In another embodiment, an rs232 port in housing  161  may be provided as an alternative programming source. Power supply  170  provides power to control logger  160  and flow meter  140 . Power supply  170  is also encased in water tight housing  171 .  
         [0023]     As shown in  FIG. 2 , collection funnel  110  may also be provided with a probe  190  to measure other environmental data. In a preferred embodiment, a Solinst 3001 LTC probe (U.S. Pat. No. 5,117,698) that continuously records tidal stage, electrical conductance, and temperature of the water within the collection funnel. In a preferred embodiment, probe  190  is a self contained module with its own control and data collection system. In addition, collection funnel  110  may be equipped with port  302  through which water within collection funnel  110  may be sampled, as further described below.  
         [0024]     In a typical application, as illustrated in  FIG. 4 , ultrasonic seepage meter  100  is installed into the bottom sediment of a marine or surface water environment. Collection funnel  110  is inserted into the bottom sediment in a marine or surface water environment, preferably to a depth of 10 cm. Funnel  110  preferably is installed so that bottom  112  is substantially parallel to the top surface of the bottom sediment, resulting in top  114  being angled slightly. This configuration and placement of funnel  110  creates a reliable seal with the sediment and reduces the chance of short-circuiting due to bottom heterogeneity or bio-irrigation from marine organisms, i.e., allows funnel  110  to effectively capture SGD. Because top  114  is angled slightly, discharge outlet  120  is slightly more elevated from the sediment bottom than the rest of funnel  110 . This allows air and/or other gases ebulliating from the sediment to escape from funnel  110  through discharge outlet  120 . By pressing funnel  110  into the sediment so that the lowest edge of top  114  is level with the sediment, headspace  118  (the open volume of funnel  110  above the sediment) is minimized, allowing for efficient flow of seepage fluid through funnel  110  and discharge outlet  120 .  
         [0025]     A field calibration is performed on the system by conducting a zero test. Valves  122  and  142  on the collection funnel  110  and the flow meter outlet  142  are both turned to the closed position. Control logger  160  is then programmed to perform a zero calibration so that travel times for the ultrasound waves are calibrated at a zero flow rate. Upon completion, valves  122  and  142  are reopened and the logger is programmed for data collection. In a preferred embodiment, the control logger is programmed to collect data at intervals from once every second to once every 24 hours. Control logger  160 , may also be programmed to collect data such as time, date, site identification, flow rate, mean flow rate, total flow rate, velocity of sound, change in arrival times, and percent aeration in the flow tube  150 . This data may be provided by sensors  180  located within flow tube  150  (see  FIG. 3 )and operatively connected to control logger  160 . Methods of doing this are well known in the art. In a preferred embodiment, flow rates are measured in units of volume per time (cm 3 /s) and a seepage velocity is obtained by dividing the calculated flow rate by the area of collection funnel  110 .  
         [0026]     The correlation between the salinity of water and the velocity f sound waves in the saline water under static flow conditions at a fixed temperature is well known. Therefore, the salinity of the groundwater discharge can be inferred from the average of the upstream and downstream speeds of the ultrasound waves through the flow tube, if the groundwater temperature is simultaneously measured. This information will assist the user in differentiating terrestrial groundwater from surrounding surface water in marine environments. Thus, the source of the discharge and any toxic or hazardous constituents present in the discharge may be more easily determined and quantified.  
         [0027]     Laboratory and field calibrations show that the ultrasonic flow meter  100  can resolve groundwater discharges on the order of 0.1 μm/s (&lt;1 cm/d), and it is sufficiently robust for deployment in the field for several days. The ability to acquire data at high rate (up to 400 times per second) allows the system to tolerate temperature density and aeration affects and still produces high-resolution accurate measurements. Flow meter  100  has also been found to be effective in measuring reverse flow rate, such as when a negative groundwater flux in which the overlying surface water is recharging the seepage zone.  
         [0028]     The present invention has been field tested and data show a relationship between seepage rate, tidal stage and hydraulic gradient from the onshore aquifer connection. This relationship results from the cyclic head changes that overlie the seepage zone and associated changes in hydraulic gradient. As tide rises, the receiving surface water hydraulic head is increasing, therefore limiting the vertical gradient between the seepage and the surface water. This leads to a decrease in the seepage flux across the sediment-water interface. As the tide is lowered, the vertical gradient begins to increase. An example data set collected in West Neck Bay, Shelter Island, N.Y. is shown in  FIG. 5 , where it can be seen that the maximum (and minimum) in tidal elevation do not correspond exactly to the minimum (and maximum) of seepage rate. In this data set, the phase lag between the tidal elevation and the seepage rate was ˜1½ hours. This was presumably due to the transient fluctuation of the water table in response to tidal loading.  
         [0029]     The present invention has the ability to be used to quantify contaminant or nutrient loading into a surface water body resulting from submarine groundwater discharge. Preferably, sampling occurs shortly after low tide corresponding to the maximum groundwater seepage rate (see  FIG. 5 ). Prior to sampling, the rate of seepage is observed. The valves  122  and  142  of both the flow meter  140  and the collection funnel  110  are closed and tygon tubing (not shown) is connected to the collection funnel  110  at port  195 . The tubing is connected to a manual peristaltic pump and water is pumped from the collection funnel  110  at a rate equal to the rate observed from the data logger prior to the closing of valves  122  and  142 . This way, measurements are taken as close as possible to actual conditions. This water is collected and taken to the laboratory for analysis. Upon completion of the sampling, the valves  122  and  142  are reopened and collection of seepage rate data may be resumed.  
         [0030]     It can be readily seen by those skilled in the art that a seepage meter in accordance with the present invention may take many different configurations in addition to the ones presented here while remaining within the spirit and scope of the present invention. Accordingly, it should be clearly understood that the embodiments of the invention described above are not intended as limitations on the scope of the invention, which is defined only by the following claims.