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The role of open source technology based equipment in developing reliable, customizable and cost-effective aquifer model for site remediation

by Campioni A., Losi P., Angelozzi I., and Santoro L. B.

Abstract

Improving the understanding of the conceptual model of a contaminated site, is key to increase the efficiency of the remediation techniques to apply. In particular, with regard to the contaminated groundwater, a reliable reconstruction of the aquifer’s structure is important to develop a numerical model of the site and identify contaminants transport pathways and fate, since the mass flux is predominant through the most permeable layers. Aquifers characterization traditionally make use of aquifer pumping tests and is often expensive and unsustainable since it involves management and disposal of large amounts of potentially contaminated water. It also requires the employment of one or more field operators to correctly carry out the tests. Therefore, the tendency is to develop tests that do not involve pumping of large volumes of water and provide in-depth details of aquifer’s characteristics.

An experimental methodology for the direct measurement of the groundwater filtration velocity and implying a low-cost, down-well electrical conductivity logging device was developed for data acquisition during a single point dilution test (SPD test). The electrical conductivity logging device used to perform the test was designed and assembled in-house in order to meet the specific needs related to the field test protocol and investigation scopes: it is lightweight, can be remotely controlled and operable by one person.

The present paper summarizes the results of a SPD test performed on a monitoring well installed in a contaminated site in Central Italy, and demonstrates the advantages of a versatile device specifically designed and built for the test needs. The device has proved to produce extremely reliable and accurate data that can be used to improve the conceptual site model for designing site remediation activities. Moreover the device can be built with minimal cost by using open source hardware and software, therefore implying more flexibility also in case of changes required to adapt the device to the particular set-up of one or more field tests.

The article also includes an evaluation of the specific technology for future enhancement of the system that can make it even more versatile and cost-effective.

Introduction

A numerical model is necessary to design effective and efficient remediation activities. It is used for understanding the hydrogeological system and its behaviour, and for predicting its response to any stress applied to it. The effectiveness of a groundwater numerical model in representing the real world is strongly dependent on the goodness of the data used to build it. Hence being able to collect reliable data in a cost-effective way is nowadays a key aspect for any remediation design.

Performing aquifer pumping tests in a contaminated environment has always posed several difficulties and risks, e.g. production of wastes, health and safety risks due to handling contaminated water, etc., while quick aquifer tests (e.g. slug tests), not involving pumping of contaminated groundwater, do not always provide reliable results. On the other hand, tracer tests often require the use of expensive equipment and many hours in the field of an experienced operator.

The following study has a twofold objective:

i. demonstrate how low-cost and open-source hardware and software can be used to design and build a field test device that provide accurate field measurements, and

ii. standardize an experimental aquifer test methodology that allows for obtaining reliable hydrogeological information in a sustainable and cost-effective manner (minimum production of wastes and minimum employment of field personnel).

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Single Point Dilution test: principles and methodology

The SPD test provides a direct estimate of the velocity of filtration (vf) in groundwater, by monitoring the course of a tracer in flows crossing a well until all signs of the tracer have completely disappeared (Settani A. and Dal Prà A., 1995). The analytical link between the velocity of filtration (vf) and tracer dilution in time, is provided by the following formula:

[1] 𝑣𝑓 = − 𝑉𝛼𝐴𝑡

𝑙𝑛 𝐶𝑡𝐶0

where V is the volume in which dilution occurs, α is a coefficient to correct the inevitable distortion of the flow net introduced by the physical structure of the well, A the section of V orthogonal to the flow lines, t is time, C0 is the concentration in the well at t=0 and Ct the concentration at time t.

From there estimating the effective groundwater velocity is just a matter of dividing Darcy velocity (vf) by effective porosity (ne):

[2] vr = vfne

and by knowing the hydraulic gradient it is possible to calculate the hydraulic conductivity (K) of the aquifer.

The test is conducted by injecting a tracer solution, taken at the same temperature as that of the groundwater, into the well, and measuring the dilution through time at different depths. To ensure a good homogenization of the solution during the injection phase, the injection tube, usually densely perforated in correspondence of the well screen interval, is repeatedly withdrawn and reinserted to the bottom of the well at a very slow and constant speed by an operator.

At the end of the injection period, the operator measures the values of rising electrical conductivity using a multiparameter probe lowered in the well and manually withdrawn to record with sampling intervals approximately every 50 cm. Measurements are taken at increasing intervals of time from the end of injection. The test is completed when the conductivity measured in the well is similar to the one measured before the injection, and the duration of a test depends on the specific characteristics of the aquifer being tested.

This procedure has some disadvantages and approximations:

homogeneous diffusion of the injected solution depends on the way the tubing has been perforated;

the operator may induce disturbance to the hydraulic conditions while lowering and withdrawing the probe, thus mixing the solution and altering the concentrations;

the operator must be always present to record EC values till the end of the test.

Custom-made electrical conductivity logging device

Down-well multiparameter probes are widely used for monitoring groundwater characteristics at contaminated sites. However, commercial instruments have a relatively high cost in the order of € 5,000.00 and are usually designed with a single probe – or a bundle of probes (e.g. temperature, ORP, DO, EC and pH) – that can take measurements at one depth at the time.

Therefore, performing a SPD test with a commercial multiparameter probe means that at least one operator shall be present for the whole duration of the test in order to lower and withdraw the device and take readings at the desired depths, and a test could potentially last for days.

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To address these limitations, in order to minimize the disturbance to the hydraulic conditions and to reduce the errors induced by manually positioning the device in the well, we decided to build a customized instrument with specific characteristics:

the physical-chemical parameters of interest are temperature and electrical conductivity, the device shall be able to measure simultaneously at different depths, the instrument shall have logging capabilities, the device shall be battery powered and easily transportable, readings shall be visualized in real-time and remotely.

The down-well part of the system is made of two data cables, one for carrying temperature probes and the other for the EC probes, each cable being 25 m long. There are 4 EC probes and as many temperature probes, one-meter spaced, with the technical specifications reported in Table 1.

Table 1 – Specifications of the sensors

Parameter Name Manufacturer Characteristics

Temperature ENV-TEMP-D AtlasScientific LLC

Temperature range: -20 °C to 133 °C Resolution: XXX.XX Accuracy: ± 1°C Submersible

Electrical Conductivity

Conductivity Probe AtlasScientific LLC

Graphite conductors Cell constant of K 0.1 Measurement range: 0.5 to 50,000 µS/cm Accuracy: ± 2 µS/cm Compensated values at 25°C Submersible

The heart of the system is represented by a Raspberry Pi model B microcomputer, connected to a 12V battery, equipped with a 3.5” TFT touch display and enclosed within a IP65 box. The Raspberry Pi is a low cost, credit-card sized Linux board with 26 General Purpose Input/Output (GPIO) pins that can be used as a physical interface between the Pi and the outside world. The Pi can be programmed in different languages (among which Python, PHP, HTML and JavaScript were chosen for the case of study) and can easily interface with actuators and sensors, like temperature and EC probes. AtlasScientific provides fully integrated interface boards for connecting its environmental probes to the Raspberry Pi, along with examples of code written in Python.

Following a preliminary testing phase (Figure 1) aimed at evaluating the functionality of the sensors and comparing the measurements with those provided by standard multiparameter probes (in the specific a Quanta-G by Hydrolab®), the system was assembled and programmed in-house.

Figure 2 is a picture of the device connected to the EC and temperature probes.

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Figure 1 – Preliminary testing phase

Figure 2 – SPDT logger connected to the probes in the field

The user interface is represented by a simple HTML page served through a light webserver running on the Pi itself, equipped with a USB WiFi dongle to provide wireless ad-hoc connectivity (SoftAP mode). The operator locally connects via WiFi to the system with a WiFi enabled device (e.g. mobile phone, computer or tablet) in order to view real-time data, set logging parameters and start/stop the system. The same operations can be performed locally through the touch display, which will display the same HTML page. In Figure 3 is an example of the user interface.

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Figure 3 – SPDT logger user interface

Total cost for the construction of the SPD logger is below € 2,000.00. An overall cost estimate can be briefly summarized as follows.

#4 EC probes, #4 temperature probes and related circuitry – € 790.00 200 m of neoprene cables and connectors – € 1,050.00 Raspberry Pi, 3.5” TFT touch display, box and miscellaneous components – € 130.00

These costs can be easily reduced by purchasing materials in bunches for at least 5 devices.

Field test

The test was conducted at a contaminated site located in Central Italy. Site characterizations activities carried out few months before, revealed the presence of high concentrations of chlorinated compounds dissolved in groundwater. The aquifer consists in a layer of gravel and sand about 8 m thick, characterized by medium to high permeability, laying on a bed of plastic clay located at approximately 10 m below ground level (bgl), with low permeability. The water table, set at about 3 m bgl, is flowing with a natural gradient of 0.35 % towards a nearby river, thus posing serious health and environmental risks.

Particularly in such cases when, due to the high permeability of the aquifer, performing standard pumping test can lead to high costs for handling contaminated purged water (considered as a waste within the Italian Environmental Framework), the SPD test represents a valid technical and economic alternative.

The test was performed on a monitoring well 10 m deep, completed with a 4” PVC pipe, and screened between 3 and 9 m bgl, as detailed in Figure 4.

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Figure 4 – Test well stratigraphic log and completion details

Prior to commence the test, a calibration solution was prepared: 35 grams of sodium chloride (NaCl, the tracer - Figure 5) were dissolved in 1 litre of groundwater purged from the test well, and the solution obtained was then progressively diluted with half litre of groundwater to obtain the correlation between the tracer concentration and the electrical conductivity (Figure 6 and Figure 7). Measurements were taken using both the low cost probe and the commercial one, in order to verify the accuracy of the system. The difference between the readings provided by the two probes is in the order of about 4%, thus confirming the reliability of the custom built device (Table 2).

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Figure 5 – Preparing the calibration solution of NaCl

Figure 6 – Field test setup

Table 2 – Correlation between tracer concentration and EC

Concentration [mg/l]

SPDT Logger Quanta-G

Temperature [°C] EC [µS/cm]1 Temperature

[°C] EC [µS/cm]

0.00 (baseline) 15.36 596 15.45 625 35,000.00 14.46 51,483 14.35 51,942 17,500.00 13.73 20,984 14.19 21,618 8,750.00 13.17 10,233 14.11 10,623 4,375.00 12.61 5,572 14.03 5,804 2,187.50 13.73 3,324 13.63 3,463 1,093.75 12.31 1,841 13.58 1,924 546.88 12.18 1,170 13.56 1,224 273.44 12.35 819 13.43 857

1 EC values are normalized to the actual temperature by using the following formula:

𝐸𝐶25 =𝐸𝐶𝑇

1 + 𝛿(𝑇 − 25)

where EC25 is the conductivity at 25°C (measured by the probe), ECT is the conductivity at temperature T and δ is a coefficient assumed equal to 0.0191°C-1 (from literature)

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Concentration [mg/l]

SPDT Logger Quanta-G

Temperature [°C] EC [µS/cm]1 Temperature

[°C] EC [µS/cm]

136.72 12.58 638 13.41 667 68.36 12.37 526 13.42 551 34.18 12.41 478 13.51 501 17.09 12.39 458 12.97 480 8.54 12.75 441 12.93 462

Figure 7 – Calibration curve

After the calibration procedure, a baseline reading was taken directly in the well by positioning the EC and temperature probes at 8, 7, 6 and 5 m below the wellhead. The logger was started with a recording interval of 20 seconds, and was left in place while the operator was preparing the tracer solution composed of about 100 litres (twice the volume of the water within the well) of a saturated solution of sodium chloride at 4,000 mg/l, corresponding to a conductivity of about 5,500 µS/cm, i.e. about 10 times the baseline value.

Injection into the test well was performed using a battery operated centrifugal pump connected to a PVC tubing (Ø 22mm) inserted into the well. Every 30 cm the injection tube was equipped with a dripper (Figure 8) auto-compensated to a maximum flow rate of 2 litres per minute.

Figure 8 – Auto-compensated dripper

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This set up allows for obtaining a good vertical homogenization of the solution without having to manually mix the solution by repeatedly withdraw and reinsert the injection tube in the well.

Injection into the test well lasted about 11 minutes at a constant flow rate of 9 l/min. During this period the level to water depth was constantly noted, and a minimal disturbance was observed in the range of few centimetres (< 2cm). EC and temperature measurements were taken and recorded at the formerly set up interval of 20 seconds, until the baseline conditions in terms of EC were reinstated. The whole test lasted about 2 hrs.

Results

Processing of experimental data to estimate the velocity of filtration was carried out starting from equation [1], which is the equation of a straight line with an angular coefficient:

[3] m = αAvfV

= kost ∙ vf

Data were plotted in semi-logarithmic form, and a value for m for each depth sampled was obtained, from which the corresponding value of vf was derived. Parameter α may generically vary between 0 and 8 (Gaspar, 1987) and in case of wells installed within granular soils without prefilter, it can be given by Ogilvi’s formula (1958):

[4] α = 4

1+(r1r2)2+ k

k1[1−(r1r2

)2]

in which r1 and r2 are the internal and external radii of the well, and k and k1 are the permeability values of the aquifer and of the filter respectively. Laboratory tests and many field investigations have in any case shown that when k1>>k, α tends to be around 2 (Gaspar, 1987).

The overall results are shown in Figure 9, while Table 3 shows values of real velocity (vr) and hydraulic permeability (K), calculated by assuming α = 2 and ne = 30%.

Table 3 – Filtration and flow velocities and hydraulic permeability

Depth [m] from wellhead vf [m/d] vr [m/d] K [m/s]

8 1.85 6.15 6.11E-03 7 1.96 6.54 6.49E-03 6 2.73 9.11 9.04E-03 5 2.27 7.55 7.49E-03

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Figure 9 – Dilution logs

Results of the test are briefly summarized below.

In line with what observed during the drilling activities, the aquifer is characterized by a high hydraulic permeability (gravel and sand).

Although according to stratigraphic logs the aquifer looks homogeneous, results of the SPD test provide a better definition and allow for identifying zones with different velocities of filtration.

The trend of dilution profile at the depth of 6 m shows that this layer, positioned at the middle of the screened interval, has the highest velocity of filtration.

Overall the tracer is rapidly and uniformly evacuated.

Conclusions and future improvements

Seeking for an alternative solution to obtain hydrogeological characteristics and minimize the production of wastes and associated risks, an experimental methodology (the single point dilution method) was approached. In order to optimize and standardize the procedure for executing such test, a customized EC-temperature logging device has been designed and developed using open-source hardware and software. This solution allowed for reducing the costs of instruments supply, minimize the hours in the field of an experienced operator, and obtain accurate and reliable data that can be used to build groundwater remediation activities.

The custom-built device can be easily adapted to different test conditions. Further improvements include

the implementation of a single cable for each sensor, allowing for positioning each probe at a desired depth and not limiting the recording interval to 1 m,

the addition of a GPRS module for complete remote connectivity, the possibility to use different probes, such as ORP, pH and DO to measure other chemical-physical

parameters.

Nowadays, given the accessibility of low-cost environmental sensors and open-source technologies, building innovative and cost-effective devices on which rely to improve the design of remediation activities is just a matter of time.

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References

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Hulla J, Ravinger R. & Turcek P. (1982) Radio-indicator tests in hydrogeological boreholes. Bulletin of Engineering Geology and the Environment Volume 26-27 n.1, 1982 pp. 439-442.

Gaspar E. (1987) Modern trends in tracer hydrology. CRC Press, Florida.

Ogilvi, N.A. (1958) Electrolytic method for the determination of the ground water filtration velocity. Bulletin of Science and Technology News, n. 4 (16), Moscow, Russia: Gosgeoltehizdat.

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