Inter-Provincial Employment Standards

We are a small construction company based in Saskatchewan but we have a contract for some work in Alberta. Whose Employment Standards should we be following? Some of our employees will be from Saskatchewan, but we are also hiring some temporary employees near the work site.  Secondarily, we have a Modified Work Arrangement that allows us to average 160 hours over 4 weeks. Will we have to re-negotiate that with our Alberta employees?
Thank you for your response!