Engineer S. ZAITSEV (Moscow)

Subriverbed, Infiltration, and Spring Waters for Water Supply and Their Sanitary Reliability in Chemical Defense (PKhO)

In a chemical attack, the contamination of water and the resulting interruption of the water supply are matters of great concern. Water sources may be contaminated by nonpersistent or persistent chemical warfare agents.

Contamination of water sources may result not only from a chemical attack but also from failure to observe mandatory rules. Such violations may include driving a contaminated vehicle into a body of water, throwing contaminated objects into it, and so forth. Therefore, all water sources must be guarded in wartime.

Different chemical warfare agents affect water to different degrees. Nonpersistent agents—chlorine, phosgene, and diphosgene—as well as persistent agents in vapor form (mustard gas, lewisite, and DIC) and smokes can contaminate water to a limited extent. Persistent liquid-droplet agents (mustard gas, arsines, lewisite, DIC, and others) poison water.

The introduction of small quantities of mustard gas and arsine into water makes it poisonous and unfit for drinking without appropriate decontamination.

Liquid-droplet agents entering water partly dissolve in it and partly settle on the bottom of the body of water, where they can persist for a very long time—in some cases for several months. As they gradually dissolve, the agents saturate the water and thereby continuously maintain its poisonous properties.

Visible signs that water sources have been contaminated with chemical warfare agents, especially soon after contamination, may include traces of the agents in the form of droplets on the soil near bodies of water, a clearly visible oily film on the surface of the water, and an odor that is particularly perceptible at night or early in the morning, when the outdoor air temperature is lower.

A day or more after contamination, these signs may be absent. The film of agent gradually dissolves and settles to the bottom, while the odor may become faint and indistinct. These circumstances are especially important.

Liquid-droplet agents can be removed from water, before decontamination, by allowing it to settle and filtering it through sand filters. For filtration, the layer of ordinary river sand in the filters must be at least 20–25 cm thick; care must also be taken to ensure that filtration proceeds uniformly and that the sand is not washed away.

In addition, ordinary crushed charcoal may be recommended for removing liquid-droplet and partially dissolved agents and their hydrolysis products from water.

We now turn to the sanitary assessment of water intakes constructed as galleries laid within riverbeds, and of individual wells receiving water…

…obtaining water by infiltration, and intake structures receiving spring water.

These three types of water intake have one feature in common: they obtain water by filtration from open bodies of water or from upper aquifers lying very close to the ground surface.

Water obtained by infiltration through intake structures in the form of galleries embedded in riverbeds, or through wells sunk into an aquifer, is often the only source of water supply, especially on railways, where it supplies steam locomotives and the inhabitants of station settlements. Spring waters, meanwhile, often supply entire cities.

These sources are very common for cities as well as for small and medium-sized military formations. We shall therefore examine their characteristics and sanitary assessment in greater detail, with particular emphasis on their significance for chemical defense (PKhO).

Table 1

Reaction of chemical warfare agents with water

The effective combat concentrations of chemical warfare agents are given in Table 2.

Table 2

Since most sources have a high yield, a considerable quantity of chemical warfare agent would be required to contaminate a water supply system.

Nonpersistent chemical warfare agents in gaseous or vapor form do not contaminate water. Persistent and semipersistent agents (such as diphosgene and chloropicrin) contaminate water in liquid form. Water contaminated with persistent agents in liquid droplets must not be used for drinking or cooking.

As a rule, decontamination of water can be supervised by chemists or sanitary physicians at the waterworks when treatment is carried out there, and by specially trained personnel when water is treated at distribution points.

The difficulty of decontaminating water, together with the inconvenience of using decontaminated water, leads to the conclusion that water must be carefully protected against contamination.

When decontaminating water containing persistent agents in liquid droplets (mustard gas, lewisite), the droplets should first be removed by settling and filtration through sand filters and charcoal, before treatment of the dissolved agent and its products begins.

At low concentrations of mustard gas contamination, water is decontaminated after settling and filtration by allowing it to stand for one hour in summer, or for three hours when the air temperature is below 5°.

While the water is standing, periodic stirring is desirable.

To accelerate decontamination, the filtered water can be brought to a boil and boiled for 5 minutes.

At high concentrations of chemical warfare agents, water is decontaminated in special installations whose operation must be controlled with special instruments and by specially trained personnel.

Although spring waters originate at great depths, their outlets are usually very close to the ground surface. In such cases, the water-bearing rock is often covered over considerable distances by a thin layer of weathered rock, which usually filters water readily. This characteristic, especially typical of descending springs, which are often situated in valleys, presents a serious risk of water contamination.

Precipitation washing over the ground surface in the catchment area can naturally wash chemical warfare agents lying on the ground surface, as well as other contaminants consisting of products of organic decomposition, into the spring water and thereby contaminate it.

Unfortunately, this serious danger is not always taken into account. Efforts are usually directed toward protecting the so-called “springhead”—more precisely, the point where the water emerges at the ground surface—while the subsurface flow upstream of the outlet is left entirely unprotected, although such protection is always necessary.

When a well-concreted or stone collecting basin is constructed at the spring’s outlet, protection of the upstream reaches is completely forgotten.

In our opinion, the sections along the flow path where it is clearly identifiable and where no impermeable strata overlie the water-bearing rock should be artificially protected. Various methods of protection are possible. The spring flow can be enclosed in a pipe or covered with impermeable material. As we have already noted, if the ground surface is sprayed with persistent chemical warfare agents, even far from the spring outlet, it can be expected that after rain the agents washed from the ground surface will penetrate the water and contaminate it. It should be emphasized that contamination of the spring may occur some time after chemical contamination of the surrounding area.

Ascending springs are in a comparatively better position, but they too can become contaminated.

Special attention should also be paid to the location and construction of the spring-capture works themselves.

The capture works must not be located in a depression, since surface water will flow toward them. The ground around the capture structures should be

…paved, with ditches dug on it (preferably interceptor ditches) to divert surface water. It is advisable, for at least a short distance from the collection point along the axis of the flow, to construct an earthen berm and ditches to prevent surface water from entering the groundwater flow.

Areas where spring waters emerge, collection structures, and areas of shallow groundwater require special protection; under no circumstances should

Fig. 1. Section along A–B.

• \(L\)
• High water level; low water level
• Drawdown curve at high water level
• Drawdown curve at low water level
• Recharge from groundwater flow, not from the open body of water
• \(I\), \(II\), \(M\), \(N\)

surface water be allowed to stagnate nearby, since during chemical attacks such water may contain persistent chemical warfare agents, such as mustard gas and lewisite.

Persistent chemical warfare agents are known to penetrate soil to a depth of 8–10 cm and melting snow to a depth of up to 20 cm, remaining there for 5 to 20 days. This particularly aggravates the danger of contamination.

Fig. 1a. Plan.

• Collection well
• Surface-water runoff
• A–B

Infiltration water is usually obtained by collecting it in wells sunk near open bodies of water: rivers, lakes, ponds, and the like.

A well may be recharged either from an open body of water (Fig. 1) along lines I–I or II–II, or from a groundwater flow that also feeds the open body of water, along line II–II.

The second case must, of course, be regarded as the most desirable if the groundwater flow supplying the well is covered by a reliable impermeable layer (usually clay) and cannot be contaminated by pollutants entering it from the ground surface.

We consider it necessary that the choice of a well site be governed by the following conditions:

1. The well should be located on ground somewhat higher than the surrounding terrain, by at least 1–2 m.
2. Surface water should flow around it on all sides, as shown in Fig. 1; the distance over which the water flows around the well should be as great as possible, preferably at least 20–25 m.
3. Surface water flowing around the well should drain away rapidly, preferably downstream along the river (water body).
4. The distance from the water’s edge at the highest water level to the well should be at least 50 m, so as not to obstruct passage or traffic and to provide a sufficiently thick filtering layer of soil.
5. The bank should have sufficient colmation to retain bacteria and other fine contaminants; it should consist of fine-grained material that rapidly retains fine suspended particles.
6. The bank should not be subject to erosion, since this destroys the colmation layer, which may be credited with the action of a biological film, as in English filters.

Fig. 2.
Diagram labels: River axis; maximum water level; siltation; gallery. Points \(А\), \(Б\), \(В\), and \(Г\); dimensions \(H_0\), \(H\), and \(h\).

Only when the above requirements are met can the intake be considered to satisfy chemical-defense and sanitary requirements.

The colmation layer, which acts as a biological film, must undoubtedly be an integral part of the entire structure. The objection that it will reduce the rate at which water can filter toward the well is not significant, because the well must be assumed to receive its water mainly through saturation of the entire body of soil surrounding it.

This must always be taken as the basis for determining the yield; the sanitary advantages justify some increase in the cost of intake structures.

We note that the colmation line, composed of dense material, lies not on the surface of the bank but at some depth within the soil, since it is formed by the deposition of suspended matter where the velocity of water flow near the bank is zero. This zero-velocity zone is usually not at the surface but somewhere within the soil mass, evidently 20–40 cm deep.

Colmation will also retain chemical warfare agents that enter the water body when it is contaminated, since such agents are known to settle out at low velocities.

Closer to the middle of the river, the possibility of chemical warfare agents settling out can be discounted because the water moves more rapidly.

Moreover, bed material—usually sand—is also transported in the middle of the river.

Water is often obtained from galleries laid in the material beneath the riverbed. The quantity of water available depends on the water saturation of the underlying stratum, the rate at which water is replenished after withdrawal from the gallery, and the composition of the filtering layer.

From a sanitary standpoint, this method of water collection calls for guidance and measures that will ensure satisfactory water quality.

To prevent chemical warfare agents and other undesirable substances from entering the collecting gallery, it is important to ensure that the water is mixed and moves rapidly enough, and to prevent displacement of the soil particles that form the filtering layer above the gallery.

From a sanitary standpoint, the following are normally of concern:

1. The depth of water continuously flowing in the river (\(H_0\)), as well as its velocity (\(v\)) at the interface between the bed and the flowing water;
2. The required thickness of the filtering layer above the gallery (\(H\));
3. Contamination of the floodplain and settlement along its banks;
4. The formation of a film, similar to a biological film, capable of retaining suspended particles and bacteria, whose presence in the gallery is unacceptable.

Surface-water runoff into the river cannot be prevented, since it is unavoidable. A number of measures should therefore be identified to preserve the sanitary quality of the water as effectively as possible.

As in the preceding case, erosion of the riverbed, and particularly the formation of scour holes, must be prevented. Galleries should be constructed only where velocities near the bed do not exceed the values at which the filtering layer is eroded (Table 3).

Table 3

Table 3 shows that the upper covering of the bed will evidently be in constant motion. It will generally consist of pebbles, since the sand will be washed away. Adhesion of chemical warfare agents and their deposition on the bed are therefore unlikely; consequently, this phenomenon may be considered a sanitary advantage of galleries.

The gallery axis should be laid out through the middle, along line A–B (Fig. 2).

According to our observations of operating galleries, a filtering layer 3–3.5 thick is sufficient.

As for the depth of water above the bed, it should be no less than 1.5 m. This will allow water to circulate in the cross-section, since otherwise there is a risk of chemical warfare agents settling out near the banks, which must not be permitted.

It should be borne in mind that clogging may also occur on the bed (along line a–b); it may develop rapidly during a flood, and deposits of chemical warfare agents may be present in it.

In view of this, every possible measure should be taken to remove the clogging layer; otherwise, contamination may penetrate the gallery and contaminate the water.

When constructing galleries beneath the beds of shallow rivers, especially mountain rivers, the sanitary inspector should give particular consideration to the points we have raised.