SUMMARY Beach sand is a habitat that supports many microbes, including viruses, bacteria, fungi and protozoa (micropsammon). The apparently inhospitable conditions of beach sand environments belie the thriving communities found there. Physical factors, such as water availability and protection from insolation; biological factors, such as competition, predation, and biofilm formation; and nutrient availability all contribute to the characteristics of the micropsammon. Sand microbial communities include autochthonous species/phylotypes indigenous to the environment. Allochthonous microbes, including fecal indicator bacteria (FIB) and waterborne pathogens, are deposited via waves, runoff, air, or animals. The fate of these microbes ranges from death, to transient persistence and/or replication, to establishment of thriving populations (naturalization) and integration in the autochthonous community. Transport of the micropsammon within the habitat occurs both horizontally across the beach, and vertically from the sand surface and ground water table, as well as at various scales including interstitial flow within sand pores, sediment transport for particle-associated microbes, and the large-scale processes of wave action and terrestrial runoff. The concept of beach sand as a microbial habitat and reservoir of FIB and pathogens has begun to influence our thinking about human health effects associated with sand exposure and recreational water use. A variety of pathogens have been reported from beach sands, and recent epidemiology studies have found some evidence of health risks associated with sand exposure. Persistent or replicating populations of FIB and enteric pathogens have consequences for watershed/beach management strategies and regulatory standards for safe beaches. This review summarizes our understanding of the community structure, ecology, fate, transport, and public health implications of microbes in beach sand. It concludes with recommendations for future work in this vastly under-studied area.

A stochastic modeling technique has been developed to investigate mass transport in a network of discrete fractures. The model is based on the repetitive generation of realizations of a fracture network from probability distributions, describing the fracture geometry, and on a solution for mass transport within each network, using a particle‐tracking technique. The system we work with consists of two orthogonal fracture sets of finite length, oriented at various angles with respect to the direction of the mean hydraulic gradient. Emphasis is placed on describing the character of dispersion, which develops as a consequence of fracture interconnectivity, and on testing the validity of the conventional diffusion‐based model of dispersion in describing transport in fractured media. Results show that mass distributions have a complex form. Marked longitudinal dispersion can develop even a short distance from a source. The distribution of mass in the direction of flow has a consistent negative skew. This pattern of dispersion arises from the limited number of pathways for mass to migrate through the network. Controlling factors in the transport process are the orientation of the fracture sets with respect to the mean hydraulic gradient, the difference in the mean flow velocity in the two fracture sets, and the standard deviation in velocity for fracture set 1. Transport patterns can change greatly as the orientation of the hydraulic gradient changes with respect to the two fracture sets. A conventional diffusion‐based model of dispersion cannot characterize transport in these fracture networks. A skewed spatial distribution of mass is observed much more frequently than a Gaussian distribution. When the mean velocities in the two fracture sets are not equal, the form of mass spreading is described by a more general, skewed distribution that accounts for the bias in the probability of mass moving along one fracture set over another. There is a tendency for mass to form a more symmetric distribution as the orientation of the two fracture sets is rotated toward a 45° angle with respect to the direction of the mean hydraulic gradient. Furthermore, constant dispersivity values or simple dispersivity functions are not definable because of the sensitivity of transport to the local velocity field in the fracture network.

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Crowe, A. S. and Smith, J. E. 2007. Distribution and persistence of DDT in soil at a sand dune-marsh environment: Point Pelee, Ontario, Canada. Can. J. Soil Sci. 87: 315-327. DDT was applied at Point Pelee, Ontario, Canada, between 1948 and1970 for mosquito control in recreational areas and pest control in former agricultural areas. Recent soil sampling programs produced 275 analyses enabling a statistical comparison of DDT concentrations with land use areas, soil conditions, and hydrologic characteristics. Concentrations of ΣDDT ranged over several orders of magnitude, with the highest concentrations (maximum 316 000 ng g -1 ) in former agricultural areas and the lowest concentrations in the natural sand dunes (maximum of 116 ng g -1 ). DDT is undergoing degradation at Point Pelee along two pathways. DDT is transformed aerobically to DDE within the sandy soils exhibiting average %DDT, %DDE and %DDD of 40%, 55%, and 5% of ΣDDT, respectively. DDT is transformed anaerobically to DDD and DDE within the marsh and flooded soils averaging 14% DDT, 44% DDE, and 42% DDD, respectively. The half-lives for the transformation of DDT to DDE within the well-drained and aerobic sandy soils at Point Pelee are highly variable and were estimated to range from 20 to 50 yr. Given the high concentrations of ΣDDT at Point Pelee and the long half-life, it is expected that DDT will remain at concentrations of concern for many decades. , on a appliqué du DDT à la pointe Pelée, une région de l'Ontario (Canada), pour lutter contre les moustiques dans les aires récréatives et pour combattre les parasites dans les anciennes zones agricoles. De récents programmes d'échantillonnage du sol ont permis 275 analyses qui devaient servir à comparer la concentration de DDT selon la vocation des terres, l'état du sol et l'hydrologie. La concentration de ΣDDT varie de plusieurs ordres de grandeur, la plus élevée (maximum de 316 000 ng/g) ayant été observée dans les vieilles zones agricoles et la plus faible, dans les dunes de sable naturelles (maximum de 116 ng/g). À la pointe Pelée, le DDT se dégrade de deux façons. Il se transforme de manière aérobie en DDE dans les sols sablonneux, avec une moyenne pour le DDT, le DDE et le DDD de 40 %, 55 % et 5 % du ΣDDT, respectivement, et de manière anaérobie en DDD et DDE dans les marais et les sols inondés avec une moyenne de 14 % de DDT, de 44 % de DDE et de 42 % de DDD, respectivement. La demi-vie nécessaire à la transformation du DDT en DDE dans les sols bien drainés et sablonneux aérobies de la pointe Pelée varie considérablement, avec une fourchette estimative de 20 à 50 ans. Étant donné la forte concentration de ΣDDT à la pointe Pelée et la longue demi-vie du DDT, on s'attend à ce que la concentration de ce pesticide demeure problématique pendant de nombreuses décennies encore.