Groundwater Quality Characterization by Means of Sampling and Use of Sensors
基于采样和在线监测地下水质量特征识别

J.Bronders

J.Bronders教授是地下水模拟和水文特征领域高级水文地质专家， 1982年毕业于地质学专业，1984年获得水文学硕士学位，1989年获得水文地质学博士学位。Bronders教授在布鲁塞尔自由大学任水文地质学家，同时也担任了多家单位的咨询专家，包括比利时和科学中心，曾经作为联合国水文学专家在斐济工作三年时间。Bronders教授于1997年任职于比利时VITO研究院，主要承担水文地质特征建模及土壤污染识别等相关工作，2008年起担任了水管理和技术团队的项目经理，主要承担水和土壤污染、雨水和土地利用相关的土地管理和经济评估的项目。2016年起陆续担任VITO环境模拟部门水河地质团队经理、业务经理。目前从事并且开拓与气候变化框架相关的动态水建模与管理相关工作。

课程内容：

As you know sampling is very important to obtain information on the status of the water. So then you can decide if water can be useful for drinking or for industry or for any other purpose. So today we will focus on the quality issues in groundwater systems. If you want to know anything about groundwater you also have to understand the hydrological system of the location where you want to know water quality. So we will focus on groundwater not on surface water like I said as for the status of the water body, surface water is a totally different hydrological science. So we will focus today on groundwater.

Next if we want to take samples from a groundwater system you need to follow standard procedures, otherwise it will be impossible to compare results. In every country and world, there are local standards which are generally the same but local changes and differences may be present. So that you have to take care always to do the sampling strategies and procedures which are in your case available in China. So today we will focus on traditional sampling which means we will take a sample before it is going to the lab. So we will focus on sampling of groundwater and last part of my presentation will be a quick view at sensors because sensors today are becoming more and more important in anything in our society, and I will give you a brief overview of some of these sensors available which you can use to monitoring groundwater quality.

The presentation is divided into four chapters which are divided into following subjects. The first thing I will try to explain to you is to give you some insights in hydrology, to have a quick view on how groundwater is present in the underground. Next I will give you an overview of

the sampling to investigate composition of groundwater. This will be a very detailed overview of all the steps you have to do and to take when you have groundwater sampling in your project. The third chapter will be on passive sampling. That’s a different type of sampling and it’ s a very useful one and we will discuss some of the benefits or the characteristics of this type of sampling. And last part will be some insight of application of sensors which might be useful in groundwater system.

General Characterization of an aquifer 含水层的一般特征

So the first thing I will explain you a bit is how geological and hydrological system is functioning. Everybody knows that groundwater is of course found in the underground. If you make a hole in the ground, in most places of the world, you can find groundwater present in the subsurface. But now groundwater is present in a Boris medium which means that the soil and the rocks are below. The underground has cracks and pause, and water is found in the pause. It is important to understand it because if you want to take a sample its necessary that you understand that the place where you are going to dig sample has enough and significant groundwater available to take a sample because not in every soil you can easily take groundwater or water samples. For example, heavy clay layer, it’s extremely difficult to pump groundwater samples from that type of soil. On the right side of my slide you see a profile in which I tried to explain the difference between layers which are Impervious, which are Aquitards, the terminology generally used for a layer which is very Impervious meaning that water is not really flowing there, and there is no water available for pumping up. It does not mean that there is no water in there. It just says that it is not available when you want to pump it. Where do we pump the water then from or take a sample? And that is the layer which is called aquifer. an aquifer layer is most of the time sandy layer and in that layer we also put our wells for example pumping drinking water and that is also the places where we can most of the time take nice samples for analytical evaluation and see how the quality of ground water is. Groundwater flow is another item which you have to understand before going to the field because the place where you want to take a groundwater sample is depending on the groundwater flow and in this little figure, I show you the water balance in natural conditions which means that water is infiltrating into the oil and is flowing towards most of the time in our nature water is ending up in a river. So that means all the places in which you want to know where the water quality is are somewhere between the river and riparian area where the water entered the groundwater system. So, you have to know where you want to take a sample and understand where the water is coming from. And if there is pollution you also the kind of system to understand where the pollution is moving to, so that you can take a

significant proper sample.

In hydrology there is only one calculation or one law which you have to understand, and that’s the Darcy’s law and its written here on the slide it explains that water is flowing from a high potential to a lower potential. In easy words, water flows from higher level to lower level. And if it is flowing into soil system there is a parameter which is called hydraulic conductivity which is the k value which is given in the equation defining how easy water can flow through your system. So if you have a layer where the k value is high, for example, in the sandy layer, then the water is flowing easily. That means if you dig a sample there, you can easily bump water from that location. So if you understand a bit of this flow equation you can generally understand and identify a nice places where you can do the sampling best.

Groundwater quality what we will talk about a little bit today. There are two different types of components present in groundwater. on the screen are those in natural condition you will find most of these elements in groundwater that are not affected by human impact because we as humans we all use chemicals which are not really present in natural conditions. So if we have a natural groundwater it is best to take samples and analyze on these components. You also see that the concentration here is given in milligrams per liter. And that’s a big difference between which we will see in the groundwater systems which are polluted there we go into that level of the micrograms which means that is much more difficult for laboratories to define the parameters or the pollutants instead of these natural components. Why is this interesting to know these differences is that there are many parameters available because we can classify types of groundwater. For example, if you are living in an area where there is a lot of calcareous rock then you will have a lot of calcium and column in the groundwater. And a system to identify these types of ground water is presented in the graph which I show on the right side of the slide called a Piper diagram where you can see all different dots representing a certain number of compositions of groundwater and you can define different groups and types of groundwater system in the areas where you have to investigate.

So it is a very important issue if you want to take samples in a natural groundwater, these are the parameters which you will look at. It doesn’t mean that in polluted areas you will not look at these parameters because they still are very useful to clarify and to identify and evaluate the groundwater quality at the site you are investigating. Most of the time we cannot enter groundwater just from above we need to install something and that the device which we install is called an observation well or a piezometer. Most of the time a pipe which is drilled by a bore hole and there is a stand pipe installed which you can schematically see on the right side of my slide. At the bottom of this construction there is a filter element, and I show you two types of filter elements, one is a metal one and the other is a PVC or plastic one. this is the devices or the location where you will take the samples so from that location you can install a pump and you can pump water and it will come in to the filter from the ground or a system, and you can a take a sample from this type of installation.

A piece of piezometer is on the right side of the slide. It is a plastic tube and that’s most of the time a PVC or another kind of plastic which has diameters between 2.5 and 10 or more centimeters, so it can be very wide. But most of the time in groundwater surveys, we need a well or observations in pipes of 5-10 centimeters. Most of the locations can be protected by a metal construction around it to protect the plastic pipe from damage. Inside of these wells it’s also possible that there are many filters installed so when you see these thing on the field and you open the cover and it is possible that there are more than one filter installed in the in the bore hole which has been constructed then it is called a multifilter-well.

Sources & Pathways of Groundwater Contamination 地下水污染的来源与途径

So next we are also interested in measuring contamination of the groundwater at most places that is why we use the piezometers and observations, because we want to ensure that pollutions which we have created in the past that they are not damaging the water quality and that we are not drinking a bad quality water. It is also important to understand where these sources of pollutions are, and it can come from many places. But if you understand where that pollution is coming from, then it is also interesting to see on which places you have to install a piezometers or where existing locations are present that you have to take samples there for contamination evaluation. As I told you contamination can come from many places, so at the moment there is a huge list of parameters which can be found in ground water and from different locations. For example, agricultural use or a waste disposal leaking or accidents. All these types of activities can create pollution. And if you doing investigation at a certain location, it is always interesting and to know in which type of parameter list or contaminants you have to look. This brings me to another concept that is used in hydrology called a conceptual model. It means that you always have to evaluate if there is a source how it will flow or how to move that’s called the pathway to a receptor. And the receptor is always the place which you want to protect. In this case in my figure here it means that I want to protect the river or I want to protect the wet lands they are indicated here as receptors. So if the pollution is coming via your ground to other system and it is ending in one of those two receptors you will have problems at that location, or you will create problems if, for example, the river is use for drinking water production. To investigate the groundwater pollution, you can always choose to install observation wells into the area. There is an example of an installed observation well we can see, if you dig sample here and it is polluted you can be sure that it is ending up to river. Of course, only one location is not enough I will show you later around that we need much more locations.

This is another type of visualization of the source spot receptor concept. In this case, there is an industrial site of pollution. If we really want to know how many locations we have to sample, it is also a very good exercise to optimize the number of observation wells which you need and where you have to do that sampling. You have to consider that the more installation you do more expensive it becomes and there is always somebody has to pay for it. So, that is why you have to understand the groundwater systems, how the flowing is and how the source spot receptor way is before you go on the field with your drilling machine and install all these observation wells which can lead to many samples. If the samples which you have taken are not useful, then it is a waste of money. So you always have to do a proper desk study before you can go on the field and installed all the items which you need for your proper ground water sampling.

Next I give an overview of a selection of parameters, because I told you that many hundreds of parameters which you should take into consideration if you go on the field and if you want to take samples and analyze them. Here I just selected a few of them to give you an idea of what this is all about. You can see the parameter groups like heavy metals and aromatic hydrocarbons and another cyanide and pesticides. An important thing here is that the values which are mentioned here are in micrograms per litter that is much lower than the milligrams per liter which I show you for the natural groundwater parameters. Another thing which is important is that these concentrations here are target values. Target value is a value which every country will have to decide for itself how high or low they want to have this, but this means if the concentration on the parameters is higher than the target value, then it is possible that you have a serious contamination. so if you have a mixture of different parameters, of course it is even more critical to look very careful to these numbers.

Sampling to Investigate Groundwater Composition地下水调查采样

The second part of the presentation which goes actually to an overview of what sampling procedures and steps you have to take before you go for example. This will be a very detailed overview which you have to take into account before you go on a field.

First of all, there should be an observation well. You have to see if they are already present or you have to decide if they have to be drilled and installed.

You need a lot of sampling equipments which in a proper condition like pumps and a few equipments you need that all in a good condition otherwise your sample will be impossible to get good analytical results. The last thing is that you have to check if you do this with local procedures and guidelines how you have to take the samples.

As I told you a piezometer which is installed in the groundwater system and you see a blue line here which says this is a static water level. This is the ground level so it means that your filter is installed in the groundwater aquifer area which is good. What you have to look is if the groundwater level is below your filter screen, and the blue line is lower than the top of the filter, you might have a problem here in this area where you have oxidization which means that your sample is affected by the oxygen content which is present in this area of the aquifer. so in most of the cases it is preferred that the filter is lower than the groundwater and the whole filter is saturated. So you can install permanent wells because you want to observe groundwater for longer systems, like if it is a contaminated site than you think you maybe next year will do a new sampling. It is better that you install a permanent well from which you can take examples every time you need and they can stay there for several years. Of course you have to take care that its maintained and that is not damaged and to check that every time.

If you only have to take one sample every ten years then it is possible that you put a temporary well. And that is not the same setup. It is a steel tube which you push into the soil. And when you are reaching a depth where you want to take a sample here that the filter is under the water level then you open the filter and take a sample. The advance is this goes fast but you can only take one sample once. It means if you remove it again and then there is nothing left on the fields so if you go back, you can never take a sample at the same place.

So if you have a good aquifer, we have to know the diameter of the filter because we need to know how much water we have to pump and how deep we have to go to take a sample. A very important issue which we always struggle with even in any part of the world is that you always have to label the piezometer and samples because otherwise you will have confusion on the field or in the lab when you come back and all your numbers are not correct that you have done a work which you have to do twice. Before you can dig samples, you have to use a pump to clean it because the water which is in the filter has been stagnated for us several time and you want to take fresh groundwater coming in. Depending on what type of a sample you want to take, you have to take into account what pollution you will expect and how deep and where the well is. And you have to take care of pumping. There are different types of pumps. Of course, I only took selection which we use for a sampling in our department and they all different and depending on what you want to pump I mean how much volume you want to pump. The peristaltic pump is the most commonly used because you can control it very stable and you can go to very low volumes. So if you want to have a very detailed and slow sampling this is the pump to use but it is normally not the pump you use to clean it because otherwise we will stay two days for pumping and that is not what you want. You want to get the sampling done in a proper way but also as quick as possible. So you have a different type of pump here which is a submersible pump which need a power supply then to make it run. There are many types of pumps and they all have advantages and disadvantages. You always have to check what you want and select your pump very carefully.

A very important item here in my presentation is the difference between a volume based sampling and low flow sampling. Low flow is a new way of sampling because you only measure the composition of a very small area around a field and it also allows you to take little volume allowing you to have a discrimination in the vertical direction. What you have to do on the field as I show you after take your equipment, the first thing to do is if the monitoring well is still in good condition. You know that where the monitoring well is located and it is in a good condition, then you measure the groundwater level. You check also if there is no free product on. Free products mean at some places you can have mineral oil floating on top of the water and then you have a problem because everything will be polluted with the oil. And that also needs special attention by pumping a sample.

Groundwater level measurement you have to take into account is that you always need a type of measuring device which you used to indicate how deep the groundwater is. First of all you want to know if there is a big change in relation to the previous measurement which is maybe six months ago and you also want to know if the filter is totally saturated which means that there is no oxidation on top of the filter and you also want to calculate from this level how much water you want to pump with your bump, otherwise you will pump not enough or you will bump too much water which is not needed. You can also have wells where you have a device which is more less a sensor measuring the depth or the height of the water.

The low flow pumping is a different story because when you bump you do not have the same kind of volume as the one which we had before, which is more interesting because you do not need large containers to take water from the site to an installation.

When you take samples on the field, it is necessary to filter the sample. For instance, using a plastic filter to get rid of small particles which are present in the water which you are pumping. Take care because it is not necessary for all samples. You always have to check with the lab

Depending on what devices they get in their laboratory or what type of parameter you have to check for. And you always have to ask the lab before you go into fields if it is needed to do a filter.

Here are some guidelines. You always need to check which location you have to sample first. If you go to de most polluted area first you risk that your equipment is contaminated. So you go from the least polluted location if you know where is the most polluted location. There are some rules like if you take the sampling tubes you do not leave them in a well, you should take them out. There are some specific rules which you have to check for the parameters like volatile parameters you have to take care that there are no air bubbles into the system. You have to take care if there in samples sometimes the laboratory needs to put a preservative, for example some acids, into your sample recipient for heavy metals. It is sometimes needed because otherwise you get oxidization and then you lose some of the information the lab will not be able to do a proper analytical evaluation. What is also important is that if you mix it too much the air will come in and it is easy to get oxidized. Put a proper number on the samples otherwise you will get mixed everything up. Then you always have to store the samples in a fridge, so that they do not get heated up during transport before they get into the lab.

Passive sampling and flux measurements 被动取样和通量测量

First item I want to tell you is the principle of solute mass flux. If you know how much kilo grams of mass is flowing to your receptor meaning that you know if the river is receiving so many kilograms of zinc for example every day, maybe that is a bigger problem than if it is not flowing. So you have to compare concentrations and mass flux. You can calculator how much pollution is moving with formula you can find them in the literature. The idea is that you know how much mass is coming in your receptor or near your receptor, depending on the type of receptor. You can decide if actions are needed or not. For example, if this pollution is coming into a river but you know that the concentration is so low that there is no reason to expect any damage. There are many locations which you have to install an observation well and you really have to look very careful how you want to manage this if you want to compare and keep the costs low. The mass flux does not replace concentration that you still have to do concentration measurements because that is the real number data which is the correct one. But it is always best to combine both and you can use the strategies of mass flux to define re mediation and that is the main advantage of mass flux determination. You can define more risk-based information with mass flux and use information for groundwater modeling to calculate and make estimation to see how the pollution is moving and you can combine modeling with measuring, and in that way, you can always optimize the actions which are needed. There are several ways to identify these mass flux. And the most used method which we use now are based on measurements in transacts and measurements of rough estimates of the volume of water which is flowing through aquifer system you can calculate many calculation but that is most of the time an estimate of the mass flux. Today there are a few companies who are making passive flux measurements and then you really know how much flux is coming through your system because you can install them in the piezometer.

Application Of Sensors (Lab on A Chip) And Real Time Measurements传感器的应用(芯片实验室)和实时测量

The last part of the presentation is on the application of sensors which is also sometimes lab on a chip which is devices doing digital analytical evaluation. Most of the sensors today do not have a lab on a chip they just give a kind of signal which allows you to do real time measurement and you can continuously measure concentration or parameters which you want to measure on the field. A sensor device is something electric or electronic. It gives a signal created by some physical quantity or chemical quantity. I want to show you the sensors giving signals related to water quality parameters meaning that the signal is influenced by chemical composition of the water. There is only a limited number of sensors which is really useful today, like electrical conductivity, pH, nitrate. And some other parameters that they are well known and used but for a typical complex parameters like pesticides and other hydrocarbons, that is not available at the moment.

I will give you quick overview. The first thing that I already show you are the divers. a diver is an interesting device to have a continuous measurement of these parameters to help you identify if there are pollutants. Another type of device is much bigger about fifty centimeters and you have a possibility to install different sensors at the bottom and these go to the well or into your surface water you can use it in surface water and groundwater and the measured parameters will be sent through your desktop to look at the data. There is also possibility to measure groundwater flow. There is a device now available which can help you to define in which direction ground water is flowing and then you can see also how much water is flowing, meaning that you also can define the discharge and you know the details of the ground water flow. The last thing we have seen rising in this kind of water monitoring is that all that data coming from sensors are going through to a database and a client or the researchers can look at the data visualization of that spot. That is interesting to do on a distance because you can follow up the conditions and you can define actions if there something wrong you can go on field and you can do the maintenance or you can check what is the problem. Traditional sampling of groundwater to provide point samples so there are only one moment while sensors allow you to do a lot of data and give you time series so you can understand dynamics and you can also use this for conceptual models and it can be targeted to do better site management.