IDENTIFYING AN “IMPACT BOUNDARY” IN A SEDIMENT TRESPASS INVESTIGATION

The phenomenon of sediment trespass has been the subject and cause of numerous litigation challenges in recent years. Down slope movement of sediment is, typically, a natural process but can be exacerbated in cases of urban development by failure to follow “best management practices.” Damage from the movement of objectionable quantities of sediment may be manifested in the in-filling of ponds, lakes, or streams or in alteration of the pre-impact topography of the land. Proving that trespass has taken place, however, may be a challenge for the geoscientist. Where non-indigenous fill material has been used at construction sites up slope, the task of identifying impact in a core of stratigraphic section is facilitated by sudden, identifiable changes in both the physical and mineralogical properties of sediments. If the up slope sediments are lithologically similar to those down slope, however, the use of particle size analysis and/or mineralogical composition is, typically, not helpful. Under such circumstances the expert witness may have to define an impact boundary by apparent cut-and-fill structures, disturbed organic horizons, or abrupt changes in the bedding characteristics of the down slope strata. An example is discussed where construction of a large shopping center caused extensive in-filling of a nearby stream. The sediment eroded from the construction site was clearly identifiable by not only its particle size characteristics but also by its contained heavy minerals. The Zircon+Tourmaline ÷ Rutile (ZTR) ratios easily identified the impact boundary, even where particle sizes of the units in question were similar.