Seed germination and root elongation tests, as a common germination index, were used to evaluate contamination of liquid and solid phases of sediments of Lake Orta (Italy). Protocols for these tests were developed after a literature survey and an investigation of the proper conditions for the germination of commercially available seeds of Cucumis sativus (cucumber), Lactuca sativa (lettuce) and Lepidium sativum (watercress). The three plant species responded differently to three types of samples, overlying water, pore water and whole sediment. The most toxic responses occurred in tests on solid samples, where the germination indices were statistically significantly decreased.
Results from tests on sediment, pore water and overlying water indicated that liming treatment has reduced pollutant loading and has improved conditions at the water-sediment interface. However, even though superficial layers of sediments were usually less toxic than those deposited in pre-liming periods, the germination indices of both Cucumis sativus and Lepidium sativum were inhibited by the sediment material of the North Basin. Since only a few centimetres of less polluted sediments cover a large stock of toxicants, resuspension, backdiffusion and/or bioturbation by oligochaetes (burrowing into sediments to a depth of 10 cm) may return some toxicants to the water column. The results obtained with this technique proved to be easy and inexpensive.