construction around the spring. Roman demands for water quality Romans preferred drinking water with a high mineral content, preferring its taste to that of soft water. Roman architect Vitruvius described the process for testing a source of drinking water: Vitruvius insisted, "Consequently we must take great care and pains in searching for springs and selecting them, keeping in view the health of mankind." The water from the Eifel aqueduct was considered to be some of the very best water in the empire. Unfortunately, hard water tends to produce calcium carbonate deposits, and all areas of the aqueduct today have a thick layer of limestone-like deposits up to thick. Despite the reduction in the cross-sectional area of the aqueduct caused by these deposits, the aqueduct was still able to provide the necessary quantity of water for Cologne. In the Middle Ages, the layer of "Eifel marble" from the aqueduct was widely reused as building material. Above-ground sections For various reasons, the Eifel aqueduct has very few above-ground sections, unlike other Roman aqueducts, such as the Pont du Gard in southern France: The course of the aqueduct was chosen so as to avoid the need for such constructions. By construction underground, the aqueduct was protected from freezing. The water arriving in Cologne had a pleasant temperature due to the insulating properties of the ground. In case of war, the aqueduct would be less easily damaged. Nonetheless, there are a few places where bridges or other constructions were necessary. The most notable was an arched bridge over the Swistbach near Rheinbach that was long and up to high. Archaeologists calculate that the original bridge had 295 arches, each wide, but the bridge has been reduced to rubble with the passage of the years. A smaller arched bridge crossed a valley near Mechernich. This was some tall and long. The archaeological remains were in good enough condition here that a partial reconstruction was built to show how the original must have looked. Roman aqueduct construction Construction of the aqueduct placed great demands on the capacities and knowledge of the Roman engineers. The Romans occasionally suffered problems of low-quality work on large projects, as witnessed by Sextus Julius Frontinus, lead official for water resources in the city of Rome, who wrote: Cost of building Considering the amount of surveying, underground building, and bricklaying involved, a construction of this size could not be built all at