Wednesday, March 23, 2011

Ground Water Table / Aquifer Recharging vs. Harvesting Rain for Reuse

This is an interesting discussion.

I would say that the these two ideas are not in opposition, but rather are complementary, in fact ideally one addresses a situation when the other has completed it's function. To use only one option would be an inefficient use of the available possibilities.

I first came across this confusion while in New Delhi, India, as I was working on some water harvesting projects there. I was presented to someone who was introduced to me as an expert on "Rainwater Harvesting", he had a PhD, and over 30 years of experience, working with the national government of the topic. We started talking and agreed that the best thing to for water management was "harvesting", it was the most practical and cost effective means of obtaining water. However when I asked him how he went about doing it, he described to me a dry well system for aquifer recharge.

 I asked him, "Okay, that's good what you do with the overflow, but what about how you harvest it?", the answers was "Yes, this is how we are harvesting rainwater."

Evidently there was a misunderstanding, we were talking about 2 different things, though related in some way, and the same name was being used for both.

The use of correct names is very important, many times we find that because names and designation have been used incorrectly then problems occur. There is a further and added complication that language or languages are dynamic entities, as technology and science evolve, new words get invented to name new things.

Not so long ago I started and created the Wikipedia article for Stormwater Harvesting, differentiating it from Rainwater Harvesting. This is a consequence of advance in technology and refinement of techniques.

Apart from this discussion of "name calling", we should be addressing what is important. I would say that the most important objective to be achieved is to have an implementation that is the most cost effective, practical and simple, to make high quality water immediately available for use with minimum, if any, power consumption. Ideally it should also be done in the most sustainable way possible.

If this is the objective, then all we have to do is find the way of achieving this objective.

To date, the most cost effective way to store water indefinitely and not only conserve it's quality, but also improve it, are in covered underground systems, that offer high surface are to volume ratios such as I have described in a previous article, where the positive of effects biomass are explained. Above ground tanks might be cheaper, however they are poor performing in terms of water quality conservation. Open water dams have problems of high level of evaporation, as well as problems with vermin and water quality degradation, from various other sources.

Someone might say though that, the ability to store water is something limited, and this is correct, it is then in this case that we should look also at aquifer or ground water recharge. Once the capacity of the underground water system has been filled then the overflow can be used in a very effective infiltration method, to recharge the water table. This method then not only has the best of both worlds, having water for immediate reuse, but also recharging the aquifer, however not only that, but if the overflow water has passed through a suitable system it is purified and goes into the aquifer it is also better for the health of the aquifer.

In recent time aquifers have suffered substantially in different aspects. It is important to replenish aquifers. Only recently have people come to realize the water is a finite resource that has to managed responsibly and with much care. It can be said that this is a similar situation to what happened with oil, what was originally though to be a resource that would last forever, became something very precious.

In the drawing below we illustrate how modern urbanisation has reduced the ability of rain or stormwater to infiltrate and enter the aquifers, not only that, but the water quality of what little infiltration occurs, is probably not very good for the aquifer as it has picked up a lot of surface contaminants. "Modern" techniques of dry wells, though give the impression of creating some good, in terms of replenishing aquifers can be more harmful than good, this is especially true in industrial areas where discharge of toxic chemicals can also enter the aquifer.
We can also see in the above drawing that in urban environments not only is there less infiltration, the infiltrated water is of worse quality, but in addition to this there is probably more demand on the aquifer, as there are probably quite a few wells in comparison to other less urbanised locations.

 The result of this continuous exploitation of aquifers with inadequate recharging has produced as a consequence the rapid descent of these, that also degrade in quality, sometimes with higher levels of salinity.

In some places such as India, this happens at an alarming rate:
The groundwater table is Delhi has depleted to 20 –30 metres in various areas across the city. Compared to a level of 30 – 40 feet at the time of Independence, the water table has dropped to 350 feet at certain places. It is said to be falling at 10 feet per year on an average. Groundwater levels have depleted by 2 – 6m in Alipur and Kanjhwla blocks, 10m ins the Najafgarh block, and about 20 m in Mehrauli block.
Some are even more dramatic in their statements, an article from the Harvard Business School Alumni Bulletin states:
"groundwater supplies in Delhi are expected to run dry by 2015"

Another aspect of concern is called seawater intrusion, this happens in more coastal areas. Significant studies have been conducted into this phenomenon:

Seawater intrusion in coastal aquifers by Food and Agriculture Organization of the United Nations. Land and Water Development Division
In relation to aquifer contamination an interesting paper is An Overview of the Current Research on Remediation of Soils and Aquifers Contaminated by Organic Compounds by C.D. Johnston and G.B. Davis.

In the figure below we can see that groundwater contamination is not something to be dealt lightly with, in fact the situation that the areas that are most vulnerable are agricultural areas indicate a very noxious cycle. 

So what is the solution? There are various, but from a consumer point of view it something that is not that complicated. In the diagram below is a simple representation of how a combined rainwater harvesting and overflow infiltration system is configured. The combination of underground tanks used to store water for immediate reuse, as well as to allow a separate tank for infiltration, that at the same time purifies the water, that is maintenance free, that requires minimum amount of power to pump the water for reuse as is practically just below the surface. Pumps for elevating and pumping ground water must be much more powerful, are as a consequence much more expensive, and in areas with rapidly descending water tables, this becomes a costly element if one has to the change the pump every few years to cope with the lowering water table.
Above we can see that with modular tanks, there is design flexibility, in relation to the amount of storage capacity for immediate reuse, as well as the infiltration tank. This way a project can harvest as much water as is necessary, conserve it ideal conditions. Water that exceeds the capacity of the tank for immediate reuse goes into the infiltration tank, and does so at an exceptional quality, and can go safely back into the aquifer.

Tuesday, March 1, 2011

How capillarity contributes to maintenance free systems

or Why do geosynthetics correctly installed permanently avoid clogging 

A frequent question I receive is related to the underground systems for water storage and management we work with, that have an envelope of geotextile, is: how is it that, the geotextile does not get clogged with silts and fines? It is an interesting question and though the easiest thing to say would be to explain that the system have been successfully used since 1986, and even before, without observations.

However, saying this sometimes is not enough, in this industry one works with a lot of engineers, and myself being an engineer, we want to know why and not only that, but have proof of it.

Though one could say that the existence of ad-hoc geotextiles have existed maybe since the first textiles where developed, our earliest human ancestors, normally used textiles to clothe themselves, or to use as vessels for carrying water or other substances. It would have probably been many years before textiles would have been available in larger enough quantities to use them for other purposes other than for essential personal use.

The interesting thing though is that I came to write about this topic, because of another reason that has nothing to do with textiles or tanks, but rather a personal hobby I have. This hobby, that I very much enjoy, is home brewing my beer. I am subscribed to Hombrewer magazine and in the Autumn 2011 edition, page 22, they had an article on particle settlement, that has precisely to do with this topic. The article titled "Clarification of Beer" explains how to determine the velocity of descent of particles in suspension, their terminal velocity and finally with this and having a distance of settlement, the time for settlement. The basic components of this system then is represented by this diagram below:
Fd = Force of drag
Fb = Force of bouyancy
Fg = Force of gravity

I found this representation quite interesting and the following formula is used in the calculations.
v = settling velocity (m/s)
g = acceleration due to gravity (9.8 m/s2)
ρ = density of water
ρs = density of particle
V = volume of particle
Cd = drag coefficient
A = projected area in the direction of motion (m2)

However it makes reference to a particle or solid in suspension that is not subject to the force of capillary action.

Capillary action is present in soil, as the spaces between granules act as capillary tubes. As such we can conclude that a more appropriate model for fine particles should change to consider this. Saying this, though we could make and endless regression where we could consider every particle in a soil profile to be under this influence, and it is... to an extent, however the difference in behaviour therefore lies in the particle densities and volumes that where mentioned above. This comparative differences are what would define "free" particles or that are in suspension, as particles that are freer to move in the soil media. The important aspect that we should consider is that this modelling is relevant for particles in suspension. As such, for example in the typical installation that we deal with, where sand is utilized, the sand is not in suspension, hence limiting the "endless" regression.

Taking the previous into account we can update our model and diagram to include the following, we can add another vector, Fc, Capillary Force, to the model as represented below.
Fc = Force of capillarity

However there is more to this than just including the concept of capillarity. Capillarity not only acts because of water cohesive force, that is manifested in waters surface tension, but also through water's adhesive force. We must remember that the difference between cohesion, that is the force of same molecules attracting to each other, and adhesion, the force of different molecules attracting to each other. As such, for our purposes, that particles should be uptaken against the force of gravity is also dependant that the suspended particle should not adhere to the particles not in suspension.

If we analyse these forces then, we can take the liberty and a assume a value of the  force of drag as negligible and the force of buoyancy as that which allows the smaller particles to be free or in suspension, but for our effects, do not enter into our equations either.

We need to then determine the force pushing upwards, that will take the particle in suspension in the same direction:

Values of surface tension can be obtained from the table below, being it interesting to note the influence on temperature, we had seen in a recent post on my blog the effect that depth positively has on the water quality, here we see it again.

Effect of temperature in degC on surface tension of water in units of mJ m-2 or mN m-1(Kaye and Laby, 1973)

Surface tension75.774.272.7571.269.6

The upwards force has to overcome the force of gravitation on the particle, which can be expressed in this way in terms of finding the point of equilibrium:

Fg = mg
We can then say:
mg = T2πr

As we want to obtain the value of r, we arrange the variable in the following way:

r = mg/T2π

As we have the value of surface tension, we need to find the mass for a "typical" particle of silt.

Specific gravity is the ratio of the weight in air of a given volume of a material at a standard temperature to the weight in air of an equal volume of distilled water at the same stated temperature.

In natural soils, particle specific gravity will usually “range numerically from 2.60 to 2.80. Within this range, the lower values for specific gravity are typical of the coarser soils, while higher values are typical of the fine-grained soil types. Values of the specific gravity outside the range of values given may occasionally be encountered in soils derived from parent materials which contained either unusually light or unusually heavy minerals.” [Ritter and Paquette 1960, p 182]

To use a worst case scenario of sediments or fines with the highest specific gravity we can consider clay or silty clay that can have specific gravities of up to 2.9, this is also heavier than quartz that has a specific gravity of 2.65, in fact most of sand is made of quartz, and as it is the media recommend for providing the capillarity and is not in suspension, we could also presume that the particles in suspension have a lower specific gravity than quartz sand.

In this case a consider what would a silt particle similar to a small sand sand grain we can assume the following, a diameter of 0.060 mm, that implies a volume of 2.51e-10 m3, and a specific gravity of 2.65 for mineral quartz, this gives a value of 0.67 mg.

If we replace the value in the equation we obtain the following:

r = (6.7e-7 * 9.8) / (72.75e-3 * 2 * 3.14)

Which provides us with the result:

 r = 14.4 μm

This can be expressed as a diameter which in the case will be 28.8 μm, which will serve a reference for pore sizes. This pore size is practically on the border of what has been defined as Mesopore and Micropores 1.

This then is the size of the capillary that provide equilibrium so that a particle smaller than the size of a grain of sand to remain in suspension. The capillary radius depends on the the particle size of the media and it has been classified in the following manner.
Pore sizes for soils can be calculated with the following table:

Soil Texture

Bulk Density (g/cm3)

Porosity (%)

Available Soil Water (inches/foot of soil depth)






  Fine Sand




  Loamy Sand




  Gravel/Cobble in Coarse Texture


Moderately Coarse
  Loamy Fine Sand




  Sandy Loam




  Fine Sandy Loam




  Gravel/Cobble in Medium Texture


  Very Fine Sandy Loam








Moderately Fine
  Sandy Clay Loam




  Silt Loam




  Clay Loam




  Sandy Clay




  Silty Clay








Peats and Mucks



A ratio is obtained by dividing  of porosity percentage with its complement, for example: in clay loam the pore space (49%) makes up almost the same amount of volume as the volume of soil particles (51%); multiply the ratio 49/51 times the 0.063 mm particle diameter to get the average pore size of 0.061 mm.

It is now interesting to see what are typical average pore sizes for soils, for example sandy loam 90μm, clay loam 61μm 2.

As the height of a capillary depends on the weight of the column of water, this is directly related to the radius of the capillary, as such the smaller the diameter of the capillary the higher the the column of water. This then produces a dichotomy in terms of that the smaller the particle size gets, eventually you get to clay sized particles that do not have much infiltration capacity and do not behave in a manner that provides capillary action.

From the above then we could conclude that the pore size we see in sandy loam, 90μm, for example is too small to provide the capillary action that we require of 28.8 μm, that we have previously determined as what would provide a capillary force that would overcome the force of gravity that the suspended particle is subject to. The situation is though that the sand, or for that case practically any soil media, is irregular and provides pore sizes that very often are much smaller that the average 90μm we calculated, as well as it providing spaces that are also larger. To illustrate this we can see the diagram below.

In another example we can see that even more variability can be represented:
Precautions necessary

In the industry we often hear of situation where supposed permeable systems become impermeable and that then need backwashing, with "sophisticated" technologies. Wouldn't it be more sophisticated to have systems that does not need maintenance or a costly backwashing system, that would not be required in the first place, if the application was designed correctly? There are many examples of these, if we were to analyse these in more details we could observe that source or cause of the clogging resides in both what we have described above, as well as other elements, that will probably include biological activity.

There are examples of these that come to mind, however to delve into them would be an odious practice.

Having analysed all of the above information what can be concluded is that if underground water management systems are installed correctly, with a suitable layer of soil media that provide capillarity, experience has shown that geotextiles do not clog.

This is possible due to the fact that it is possible to infer that there are spaces where capillary radii are smaller than those required to provide enough force to overcome gravity, that it in fact acts, literally, as a micro physical mechanism that provides a cleaning action that maintains the geotextile clean and free of clogging.


Soil analysis: an interpretation manual By Kenneth Ian Peverill, L. A. Sparrow, Douglas J. Reuter, 96
T.J. Marshall, J.W. Holmes, and C.W. Rose (1996), Soil Physics, 3rd Edition, 42-45