In Italian the word 'conca' is most typically used in geography, where it translates as 'basin', 'dell', 'sinking' (see http://www.ancestry.com/facts/Conca-family-history.ashx
). Now, in Italy there is a number of places named 'Conca ...' or '... Conca', such as Conca Casale (province of Isernia, Molise), Conca dei Marini (province of Salerno, Campania), Conca della Campania (province of Caserta, Campania), Mercatino Conca (province of Pesaro and Urbino, Marche), Montefiore Conca (province of Rimini, Emilia Romagna), etc: all these place names seem to support a topographic origin for the surname Conca.
Curiously enough, though, the word 'conca' also means 'head' in some Italian dialects, most of all in Sardinian - Sardinia island has a big number of Concas families ('concas' is plural of 'conca' in Sardinian), though the number of Conca families is minor in the same region. Moreover, a lot of Italian surnames have the meaning of 'head' (the clearest example is Testa), so there are some chances that Conca is one of those surnames too - that might be a partial source for the surname at least.