ABSTRACT
Pretreated Rhine River water is infiltrated in Amsterdam’s dune catchment area, through 40 basins. It is recovered by a drainage system and recovery canals. Sludges accumulate in recharge basins, recovery canals and natural dune lakes. Recharge sludges are subdivided into old (B1), young (B2) and intermediate (B). Type Bl formed before 1974, when pretreatment was simple. It shows the highest accumulation rate (2 cm/y) and pollution index of all. Type B2 formed after 1974, when coagulation was added to the pretreatment. It shows the lowest accumulation rate (0.2 cm/y), least organic material, As, Cd, Ni and PAHs, and a low pollution index. Recovery sludges (4 subtypes; 0.4–1.1 cm/y) have the highest Fe and As content (leaching of aquifer) and lowest Cu and Zn content (retention by aquifer). Natural sludges from dune lakes (0.7 cm/y) are least polluted of all, contain least EOX, but most organic material. Recommendations are given on how to reduce sludge formation and improve its quality.
