requires list of these substances as data.frame for each site (one column per site). If substance is always < dl at one site, results are set to zero. If substance is sometimes > dl, resluts are set to a factor*dl
adapt_nondetect(x_in, x_nd, factor = 0.5)name of input data.frame
vector with substances < detection (one column per site)
multiplier of detection limit if smaller dl (e.g., 0, 0.5 or 1)