introduCtion Exterior Acid Attack Although the chemistry is the same as for an interior attack, an exterior acid attack involves a different set of conditions. The parameters are more complicated and less predictable, and larger variations in the rate of the replenishment can be expected. However, the maximum rate of replenishment, due to the flow through soil materials, will be much slower than that encountered with an acidic effluent, with the rate frequently being almost negligible. For these conditions, pH is not the only measure needed to evaluate aggressiveness. Since pH does not measure how much acid is available to be neutralized, total acidity, normally expressed in terms of milligram equivalents of acid per 100 grams of soil, must be evaluated. If the total acidity is low, a very low pH can be readily increased by a small alkaline neutralization. A meaningful description of the acidity of any given soil or groundwater must include both pH and total acidity. Of the two, total acidity is more significant, provided that the groundwater in contact with the pipe is relatively static. The greater the anticipated movement of groundwater, the greater is the replenishment rate and the significance of pH. Measurements of pH can be accurately made in the field with a standard pH meter. Samples of soil or groundwater to be tested should be taken from the level of the proposed pipeline, as samples taken from the surface are meaningless and can be misleading. Tests should be made at regular intervals along the proposed route. If hot spots, or high acidic conditions, are encountered, their extent should be determined and a number of measurements made within the suspect area. An effective or realistic design value can be determined by a statistical evaluation of a series of soil pH readings. One, or even a few, scattered low pH values would not establish a reasonable design value. The same soil samples can be tested for total acidity using standard methods, and evaluated statistically and in conjunction with pH. As a guideline, a pH of 5 and a total acidity of 25 milligram equivalents per 100 grams of soil indicates a potentially aggressive groundwater situation. Such conditions require a more comprehensive analysis of installation characteristics to determine if countermeasures would be necessary to ensure durability. Naturally occurring groundwaters with this degree of acidity are quite rare. Man-made conditions, such as sanitary landfills and industrial waste disposal areas, are the most likely areas where this combination or greater might be encountered. ConCrete pipe stormwater treatment teChnologies box seCtions manhole and CatCh basin material LAFARGE PIPE eMANUAL PAGE 49