Academic journal article Journal of Environmental Health

Problems Associated with the Use of Gravel in Septic-Tank Leachfield Systems

Academic journal article Journal of Environmental Health

Problems Associated with the Use of Gravel in Septic-Tank Leachfield Systems

Article excerpt

Introduction

The widespread use of leaching systems without gravel in the last decade has focused the regulatory community's attention on the sizing factors that should be assigned to these systems. During this debate, a far more serious issue - the potential and measured impact of gravel on the performance of conventional leaching systems - has been obscured. These impacts have been observed and measured in direct laboratory and field studies, and can be inferred from comparative performance evaluation. Further, an examination of well-understood engineering principles demonstrates that gravel is poorly adapted to the role in which it has been placed. The impact that gravel has on any specific system's performance varies greatly with a variety of factors. These include cleanliness and specifications of gravel, method of placement, host soil moisture content, host soil type, climatic conditions, wastewater strength, method of application, and system age.

It is impossible to compare a very small system sample to a system in actual field use and state that the sample represents the exact negative effects of gravel in the field. This is especially true when gravel is used with special care in an experimental setting which may not simulate field conditions. To determine the actual effect, the entire body of evidence on the subject must be considered.

It should also be understood that several program management decisions substantially affect the degree of gravel's impact. The first of these decisions is the method of determining minimum system sizes. This varies considerably from state to state. Many sizing requirements for gravel-filled systems are based on studies which measure infiltration rates without gravel (1). Despite the fact that several critical researchers pointed out that gravel would have a negative impact, the application rates developed from their work were used in gravel-filled systems (2).

The other critical aspect of the management issue is the perception that existing system sizes are adequate. In the author's experience within the State of Connecticut and in several highly significant studies in North Carolina, almost all regulators indicate that their system failure rate falls below 5% (3). This is at considerable variance with actual observed failure rates identified in several studies (3,4,5). In these studies, failure rates of relatively new ([less than]6 year old) septic-tank leachfield systems using conventional gravel and gravity flow distribution, ranged from 11% to 34%. At least one other study suggests an even higher rate of failure (4). The negative impacts of gravel and the impacts of those factors on leachfield size comprise only a portion of the problems that cause premature failure. Another contributing factor is that the design and regulatory community remains largely uninformed about actual failure rates. This has prevented careful reexamination of fundamental issues, such as the negative impact of gravel on actual system performance. These factors are increasingly important in view of findings that much of our current system sizing is not as conservative as most in the regulatory community have assumed (6).

Methods

The negative impacts of gravel can be examined from a variety of standpoints, including a summary of engineering properties that make gravel less than ideal for leaching systems, a summary of research that has directly measured the negative impacts of gravel from either a physical or biological standpoint, and a summary of field data that supports the small-scale measurements of these phenomena. Full consideration of each of these factors and studies is beyond the scope of this paper and they are therefore dealt with in an abbreviated fashion. The assembled data and studies form a thesis that is unusually well supported in the field of on-site wastewater disposal.

Discussion

Summary of Problematic Engineering Properties of Gravel

The following section reviews the fundamentals used by engineers or scientists involved with soils engineering. …

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